Semiconductor architecture having field-effect transistors especially suitable for analog applications

ABSTRACT

An insulated-gate field-effect transistor ( 100, 100 V,  140, 150, 150 V,  160, 170, 170 V,  180, 180 V,  190, 210, 210 W,  220, 220 U,  220 V,  220 W,  380 , or  480 ) has a hypoabrupt vertical dopant profile below one ( 104  or  264 ) of its source/drain zones for reducing the parasitic capacitance along the pn junction between that source/drain zone and adjoining body material ( 108  or  268 ). In particular, the concentration of semiconductor dopant which defines the conductivity type of the body material increases by at least a factor of 10 in moving from that source/drain zone down to an underlying body-material location no more than 10 times deeper below the upper semiconductor surface than that source/drain zone. The body material preferably includes a more heavily doped pocket portion ( 120  or  280 ) situated along the other source/drain zone ( 102  or  262 ). The combination of the hypoabrupt vertical dopant profile below the first-mentioned source/drain zone, normally serving as the drain, and the pocket portion along the second-mentioned source/drain zone, normally serving as the source, enables the resultant asymmetric transistor to be especially suitable for high-speed analog applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. patent application Ser. No. 12/940,000, filed4 Nov. 2010, now U.S. Pat. No. 8,013,390 B1, which is a division of U.S.patent application Ser. No. 12/545,024, filed 20 Aug. 2009, now U.S.Pat. No. 7,863,681 B1, which is a division of U.S. patent applicationSer. No. 11/981,481, filed 31 Oct. 2007, now U.S. Pat. No. 7,642,574 B2,which is a continuation-in-part of U.S. patent application Ser. No.11/977,213, filed 23 Oct. 2007, now U.S. Pat. No. 7,838,930 B1, which isa division of U.S. patent application Ser. No. 11/215,537, filed 29 Aug.2005, now U.S. Pat. No. 7,419,863 B1. This is also related to U.S.patent application Ser. No. 11/981,355, filed 31 Oct. 2007, now U.S.Pat. No. 7,838,369 B2. All of the material in U.S. application Ser. Nos.11/215,537 and 11/981,355 is incorporated by reference herein to theextent not expressly repeated herein.

FIELD OF USE

This invention relates to semiconductor technology and, in particular,to field-effect transistors (“FETs”) of the insulated-gate type. All ofthe insulated-gate FETs (“IGFETs”) described below are surface-channelenhancement-mode IGFETs except as otherwise indicated.

BACKGROUND

An IGFET is a semiconductor device in which a gate dielectric layerelectrically insulates a gate electrode from a channel zone extendingbetween a source zone and a drain zone. The channel zone in anenhancement-mode IGFET is part of a body region, often termed thesubstrate or substrate region, that forms respective pn junctions withthe source and drain. In an enhancement-mode IGFET, the channel zoneconsists of all semiconductor material between the source and drain.During IGFET operation, charge carriers move from the source to thedrain through a channel induced in the channel zone along the uppersemiconductor surface. The threshold voltage is the value of thegate-to-source voltage at which the IGFET switches between its on andoff states for given definitions of the on and off states. The channellength is the distance between the source and drain along the uppersemiconductor surface.

IGFETs are employed in integrated circuits (“ICs”) to perform variousdigital and analog functions. As IC operational capabilities haveadvanced over the years, IGFETs have become progressively smaller,leading to a progressive decrease in minimum channel length. An IGFETthat operates in the way prescribed by the classical model for an IGFETis often characterized as a “long-channel” device. An IGFET is describedas a “short-channel” device when the channel length is reduced to suchan extent that the IGFET's behavior deviates significantly from theclassical IGFET model. Although both short-channel and long-channelIGFETs are employed in ICs, the great majority of ICs utilized fordigital functions in very large scale integration applications are laidout to have the smallest channel length reliably producible withavailable lithographic technology.

A depletion region extends along the junction between the source and thebody region. Another depletion region extends along the junction betweenthe drain and the body region. A high electric field is present in eachdepletion region. Under certain conditions, especially when the channellength is small, the drain depletion region can laterally extend to thesource depletion region and merge with it below the upper semiconductorsurface. This phenomenon is termed (bulk) punchthrough. Whenpunchthrough occurs, the operation of the IGFET cannot be controlledwith its gate electrode. Punchthrough needs to be avoided.

Various techniques have been employed to improve the performance ofIGFETs, including those operating in the short-channel regime, as IGFETdimensions have decreased. One performance improvement techniqueinvolves providing an IGFET with a two-part drain for reducinghot-carrier injection. The IGFET is also commonly provided with asimilarly configured two-part source.

FIG. 1 illustrates such a conventional long n-channel IGFET 20 asdescribed in U.S. Pat. No. 6,548,842 B1 (Bulucea et al.). The uppersurface of IGFET 20 is provided with recessed electrically insulatingfield-insulating region 22 that laterally surrounds active semiconductorisland 24 having n-type source/drain (“S/D”) zones 26 and 28. Each S/Dzone 26 or 28 consists of very heavily doped main portion 26M or 28M andmore lightly doped, but still heavily doped, lateral extension 26E or28E.

S/D zones 26 and 28 are separated from each other by channel zone 30 ofp-type body material 32 consisting of lightly doped lower portion 34,heavily doped intermediate well portion 36, and upper portion 38.Although most of upper body-material portion 38 is moderately doped,portion 38 includes ion-implanted heavily doped halo pocket portions 40and 42 that respectively extend along S/D zones 26 and 28. IGFET 20further includes gate dielectric layer 44, overlying gate electrode 46,electrically insulating gate sidewall spacers 48 and 50, and metalsilicide layers 52, 54, and 56.

S/D zones 26 and 28 are largely mirror images of each other. Halo pocketportions 40 and 42 are also largely mirror images of each other so thatchannel zone 30 is symmetrically longitudinally graded with respect tochannel dopant concentration. As a result, IGFET 20 is a symmetricdevice. Either S/D zone 26 or 28 can act as source during IGFEToperation while the other S/D zone 28 or 26 acts as drain. This isespecially suitable for digital situations where S/D zones 26 and 28respectively function as source and drain during certain time periodsand respectively as drain and source during other time periods.

FIG. 2 illustrates how net dopant concentration N_(N) varies as afunction of longitudinal distance x for IGFET 20. Since IGFET 20 is asymmetric device, FIG. 2 presents only a half profile starting from thechannel center. Curve segments 26M*, 26E*, 28M*, 28E*, 30*, 40*, and 42*in FIG. 2 respectively represent the net dopant concentrations ofregions 26M, 26E, 28M, 28E, 30, 40, and 42. Dotted curve segment 40″ or42″ indicates the total concentration of the p-type dopant that formshalo pocket 40 or 42, including the p-type dopant introduced into thelocation for S/D zone 26 or 28 in the course of forming pocket 40 or 42.

In addition to helping alleviate undesired roll off of the thresholdvoltage at short channel length, the presence of halo pockets 40 and 42in IGFET 20 causes the net p-type dopant concentration in channel zone30 to be increased along each S/D zone 26 or 28, specifically along eachlateral extension 26E or 28E. The onset of punchthrough is therebyalleviated because the thickness of the channel-zone portion of thedepletion region extending along the junction of source-acting S/D zone26 or 28 is reduced.

Body material 30 is provided with an additional doping characteristic tofurther alleviate punchthrough. Based on the information presented inU.S. Pat. No. 6,548,842 B1, FIG. 3 a roughly depicts how absoluteconcentrations N_(T) of the p-type and n-type dopants vary as a functionof depth y along a vertical line extending through main S/D portion 26Mor 28M as a result of the additional doping characteristic. Curvesegment 26M″ or 28M″ in FIG. 3 a represent the total concentration ofthe n-type dopant that defines main S/D portion 26M or 28M. Curvesegments 34″, 36″, 38″, 40″, and 42″ together represent the totalconcentration of the p-type dopant that defines respective regions 34,36, 38, 40, and 42.

The additional doping characteristic is achieved by ion implantingp-type upper body-material portion 38 with p-type anti-punchthrough(“APT”) dopant that reaches a maximum concentration at a depth more than0.1 μm below the upper semiconductor surface but no more than 0.4 μmbelow the upper surface. For the situation represented in FIG. 3 a wheremain S/D portions 26M and 28M extend approximately 0.2 μm below theupper surface, the p-type APT dopant reaches a maximum concentration ata depth of approximately 0.2 μm. By locating the p-type APT dopant inthis manner, the thickness of the channel-zone portion of the depletionregion extending along the pn junction of source-acting S/D zone 26 or28 is further reduced so as to further alleviate punchthrough.

Well region 36 is defined by ion implanting IGFET 20 with p-type welldopant that reaches a maximum concentration at a depth below that of themaximum concentration of the p-type APT dopant. Although, the maximumconcentration of the p-type well dopant is somewhat greater than themaximum concentration of the p-type APT dopant, the vertical profile ofthe total p-type dopant is relatively flat from the location of themaximum well-dopant concentration up to main S/D portion 26M or 28M. Inparticular, N_(T) concentration of the total p-type dopant decreases byconsiderably less than a factor of 5 in going from the location of themaximum well-dopant concentration up to main S/D portion 26M or 28M.

U.S. Pat. No. 6,548,842 B1 discloses that the p-type dopant profilealong the above-mentioned vertical line through main S/D portion 26M or28M can be further flattened by implanting an additional p-type dopantthat reaches a maximum concentration at a depth between the depths ofthe maximum concentrations of APT and well dopants. This situation isillustrated in FIG. 3 b for such a variation of IGFET 20 where curvesegment 58″ indicates the variation caused by the further p-type dopant.In FIG. 3 b, the maximum concentration of the further p-type mm dopantlies between the maximum concentrations of the APT and well dopants.Accordingly, concentration N_(T) of the total p-type dopant againdecreases by considerably less than a factor of 5 in moving from thelocation of the maximum well-dopant concentration to portion 26M or 28M.

A symmetric IGFET structure is not needed in situations, especially manyanalog applications, where current flows in only one direction throughan IGFET during device operation. As further discussed in U.S. Pat. No.6,548,842 B1, the halo pocket portion can be deleted from the drainside. IGFET 20 thereby becomes long N-channel IGFET 60 as shown in FIG.4 a. IGFET 60 is an asymmetric device because channel zone 30 isasymmetrically longitudinally dopant graded. S/D zones 26 and 28 inIGFET 60 respectively function as source and drain. FIG. 4 b illustratesasymmetric short n-channel IGFET 70 corresponding to long-channel IGFET60. In IGFET 70, source-side halo pocket 40 closely approaches drain 28.Net dopant concentration N_(N) as a function of longitudinal distance xalong the upper semiconductor surface is shown in FIGS. 5 a and 5 brespectively for IGFETs 60 and 70.

Asymmetric IGFETs 60 and 70 receive the same APT and well implants assymmetric IGFET 20. Along vertical lines extending through source 26 anddrain 28, IGFETs 60 and 70 thus have the dopant distributions shown inFIG. 3 a except that dashed-line curve segment 62″ represents thevertical dopant distribution through drain 28 due to the absence of halopocket 42. When the IGFET structure is provided with the additional wellimplant to further flatten the vertical dopant profile, FIG. 3 bpresents the consequent vertical dopant distributions again subject tocurve segment 62″ representing the dopant distribution through drain 28.

U.S. Pat. Nos. 6,078,082 and 6,127,700 (both Bulucea) describe IGFETshaving asymmetric channel zones but different vertical dopantcharacteristics than those employed in the inventive IGFETs of U.S. Pat.No. 6,548,842 B1. IGFETs having asymmetric channel zones are alsoexamined in other prior art documents such as (a) Buti et al.,“Asymmetrical Halo Source GOLD drain (HS-GOLD) Deep Sub-half n-MicronMOSFET Design for Reliability and Performance”, IEDM Tech. Dig., 3-6Dec. 1989, pp. 26.2.1-26.2.4, (b) Chai et al., “A Cost-Effective 0.25 μmL_(eff) BiCMOS Technology Featuring Graded-Channel CMOS (GCMOS) and aQuasi-Self Aligned (QSA) NPN for RF Wireless Applications”, Procs. 2000Bipolar/BiCMOS Circs. and Tech. Meeting, 24-26 Sep. 2000, pp. 110-113,(c) Cheng et al., “Channel Engineering for High Speed Sub-1.0 V PowerSupply Deep Submicron CMOS”, 1999 Symp. VLSI Tech., Dig. Tech. Paps.,14-16 Jun. 1999, pp. 69 and 70, (d) Deshpande et al., “ChannelEngineering for Analog Device Design in Deep Submicron CMOS Technologyfor System on Chip Applications,” IEEE Trans. Elec. Devs., September2002, pp. 1558-1565, (e) Hiroki, “A High Performance 0.1 μm MOSFET withAsymmetric Channel Profile”, IEDM Tech. Dig., December 1995, pp.17.7.1-17.7.4, (f) Lamey et al., “Improving Manufacturability of an RFGraded Channel CMOS Process for Wireless Applications”, SPIE Conf.Microelec. Dev. Tech. II, September 1998, pp. 147-155, (g) Ma et al.,“Graded-Channel MOSFET (GCMOSFET) for High Performance, Low Voltage DSPApplications”, IEEE Trans. VLSI Systs. Dig., December 1997, pp. 352-358,(h) Matsuki et al., “Laterally-Doped Channel (LDC) Structure forSub-Quarter Micron MOSFETs”, 1991 Symp. VLSI Tech., Dig. Tech. Paps.,28-30 May 1991, pp. 113 and 114, and (i) Su et al., “A High-PerformanceScalable Submicron MOSFET for Mixed Analog/Digital Applications”, IEDMTech. Dig., December 1991, pp. 367-370.

The term “mixed signal” refers to ICs containing both digital and analogcircuitry blocks. The digital circuitry typically employs the mostaggressively scaled n-channel and p-channel IGFETs for obtaining themaximum potential digital speed at given current leakage specifications.The analog circuitry utilizes IGFETs and/or bipolar transistorssubjected to different performance requirements than the digital IGFETs.Requirements for the analog IGFETs commonly include high linear voltagegain, good small-signal and large-signal frequency response at highfrequency, good parameter matching, low input noise, well controlledelectrical parameters for active and passive components, and reducedparasitics, especially reduced parasitic capacitances. Although it wouldbe economically attractive to utilize the same transistors for theanalog and digital blocks, doing so would typically lead to weakenedanalog performance. Many requirements imposed on analog IGFETperformance conflict with the results of digital scaling.

More particularly, the electrical parameters of analog IGFETs aresubjected to more rigorous specifications than the IGFETs in digitalblocks. In an analog IGFET used as an amplifier, the output resistanceof the IGFET needs to be maximized in order to maximize its intrinsicgain. The output resistance is also important in setting thehigh-frequency performance of an analog IGFET. In contrast, the outputresistance is considerably less importance in digital circuitry. Reducedvalues of output resistance in digital circuitry can be tolerated inexchange for higher current drive and consequent higher digitalswitching speed as long as the digital circuitry can distinguish itslogic states, e.g., logical “0” and logical “1”.

The shapes of the electrical signals passing through analog transistorsare critical to circuit performance and normally have to be maintainedas free of harmonic distortions and noise as reasonably possible.Harmonic distortions are caused primarily by non-linearity of transistorgain and transistor capacitances. Hence, linearity demands on analogtransistors are very high. The parasitic capacitances at pn junctionshave inherent voltage non-linearities that need to be alleviated inanalog blocks. Conversely, signal linearity is normally of secondaryimportance in digital circuitry.

The small-signal analog speed performance of IGFETs used in analogamplifiers is determined at the small-signal frequency limit andinvolves the small-signal gain and the parasitic capacitances along thepn junctions for the source and drain. The large-signal analog speedperformance of analog amplifier IGFETS is similarly determined at thelarge-signal frequency limit and involves the non-linearities of theIGFET characteristics.

The digital speed of logic gates is defined in terms of the large-signalswitching time of the transistor/load combination, thereby involving thedrive current and output capacitance. Hence, analog speed performance isdetermined differently than digital speed performance. Optimizations foranalog and digital speeds can be different, leading to differenttransistor parameter requirements.

Digital circuitry blocks predominantly use the smallest IGFETs that canbe fabricated. Because the resultant dimensional spreads are inherentlylarge, parameter matching in digital circuitry is often relatively poor.In contrast, good parameter matching is usually needed in analogcircuitry to achieve the requisite performance. This typically requiresthat analog transistors be fabricated at greater dimensions than digitalIGFETs subject to making analog IGFETS as short as possible in order tohave source-to-drain propagation delay as low as possible.

In view of the preceding considerations, it is desirable to have asemiconductor architecture that provides IGFETs with good analogcharacteristics. The analog IGFETs should have high intrinsic gain, highoutput resistance, high small-signal speed with reduced parasiticcapacitances, especially reduced parasitic capacitances along the sourceand drain junctions. It is also desirable that the architecture becapable of providing high-performance digital IGFETs.

GENERAL DISCLOSURE OF THE INVENTION

The present invention provides such an architecture. In accordance withthe invention, a semiconductor structure contains a principal IGFEThaving comparatively low parasitic capacitance along at least one of thepn junctions that form source/drain boundaries. Although usable indigital applications, the principal IGFET is particularly suitable foranalog applications and can achieve excellent analog performance.

The semiconductor structure of the invention may include an additionalIGFET configured similar to, but of opposite polarity to, the principalIGFET. The two IGFETs thereby form a complementary-IGFET architectureespecially useful for analog circuitry. The present semiconductorstructure may also contain a further IGFET, or two furtheropposite-polarity IGFETs, particularly suitable for digital circuitry.The overall architecture can then be employed in mixed-signal ICs.

Returning to the principal IGFET, it contains a channel zone, a pair ofsource/drain (“S/D”) zones, a gate dielectric layer overlying thechannel zone, and a gate electrode overlying the gate dielectric layerabove the channel zone. The principal IGFET is created from asemiconductor body having body material of a first conductivity type.The channel zone is part of the body material and thus is of the firstconductivity type. The S/D zones are situated in the semiconductor bodyalong its upper surface and are laterally separated by the channel zone.Each S/D zone is of a second conductivity type opposite to the firstconductivity type so as to form a pn junction with the body material.

A well portion of the body material extends below the S/D zones. Thewell portion is defined by semiconductor well dopant of the firstconductivity type and is more heavily doped than overlying andunderlying portions of the body material. Importantly, the concentrationof the well dopant reaches a principal subsurface maximum along alocation no more than 10 times deeper, preferably no more than 5 timesdeeper, below the upper semiconductor surface than a specified one ofthe S/D zones. This enables the concentration of all dopant of the firstconductivity type in the body material to decrease by at least a factorof 10, preferably at least a factor of 20, in moving upward from thelocation of the subsurface maximum in the well dopant's concentration tothe specified S/D zone.

Alternatively stated, the concentration of all dopant of the firstconductivity type in the body material increases at least 10 times,preferably at least 20 times, in moving from the specified S/D zonedownward to a body-material location no more than 10 times deeper,preferably no more than 5 times deeper, below the upper semiconductorsurface than that S/D zone. This subsurface body-material locationnormally lies below largely all of each of the channel and S/D zones. Byproviding the body material with this “hypoabrupt” dopant distribution,the parasitic capacitance along the pn junction between the bodymaterial and the specified S/D zone is comparatively low. The principalIGFET can thus achieve high analog performance.

The principal IGFET is normally an asymmetric device in that the channelzone is asymmetrically longitudinally dopant graded. Specifically, theconcentration of the dopant of the first conductivity type in the bodymaterial is lower where the channel zone meets the specified S/D zonealong the upper semiconductor surface than where the channel zone meetsthe remaining one of the S/D zones along the upper surface. Thespecified S/D zone then normally constitutes the drain during IGFEToperation while the remaining S/D zone constitutes the source. Theconcentration of the dopant of the first conductivity type in the bodymaterial is normally at least a factor of 10 lower, preferably at leasta factor of 20 lower, where the channel zone meets the drain along theupper surface than where the channel zone meets the source along theupper surface. Alternatively stated, the concentration of the dopant ofthe first conductivity type in the body material is normally at least 10times higher, preferably at least 20 times higher, where the channelzone meets the source along the upper surface than where the channelzone meets the drain along the upper surface.

The high dopant concentration along the source side of the channel zoneshields the source from the comparatively high electric field in thedrain because the electric field lines from the drain terminate onionized dopant atoms which are situated in the channel zone near thesource and which provide the higher channel-zone dopant concentrationnear the source rather than terminating on ionized dopant atoms in thedepletion region along the source and detrimentally lowering theabsolute value of the potential barrier for majority charge carrierscoming from the source. This alleviates punchthrough. The combination ofthe above-mentioned hypoabrupt vertical dopant profile below thespecified S/D zone, i.e., the drain here, and the increased channel-zonedopant concentration at the source side can thereby achieve high analogperformance without punchthrough failure.

In short, the present invention furnishes a semiconductor architecturehaving an IGFET, or a pair of opposite-polarity IGFETs, especiallysuitable for analog circuitry. The present architecture may include afurther IGFET, or a pair of opposite-polarity further IGFETs, especiallysuitable for digital circuitry. The resultant architecture can handlemixed-signal applications very well. The invention thereby provides asubstantial advance over the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front cross-sectional view of a prior art symmetric longn-channel IGFET.

FIG. 2 is a graph of net dopant concentration along the uppersemiconductor surface as a function of longitudinal distance from thechannel center for the IGFET of FIG. 1.

FIGS. 3 a and 3 b are graphs of absolute dopant concentration as afunction of depth along vertical lines through the source/drain zones attwo respective different well-doping conditions for the IGFETs of FIGS.1, 4 a, and 4 b.

FIGS. 4 a and 4 b are front cross-sectional views of respective priorart asymmetric long and short n-channel IGFETs.

FIGS. 5 a and 5 b are graphs net dopant concentration along the uppersemiconductor surface as a function of longitudinal distance from thechannel center for the respective IGFETs of FIGS. 4 a and 4 b.

FIG. 6 is a front cross-sectional view of an asymmetric long n-channelIGFET configured according to the invention so as to have asemiconductor well portion of the same conductivity type as directlyunderlying semiconductor material.

FIGS. 7 a-7 c are respective graphs of individual, absolute, and netdopant concentrations as a function of longitudinal distance along theupper semiconductor surface for the IGFET of FIG. 6, 18 a, 68 a, or 68b.

FIGS. 8 a-8 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical linethrough the source of the IGFET of FIG. 6, 11, or 13.

FIGS. 9 a-9 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a pair of verticallines through the channel zone of the IGFET of FIG. 6, 11, 13, or 15.

FIGS. 10 a-10 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical linethrough the drain of the IGFET of FIG. 6, 11, 13, 18 a, or 18 b.

FIG. 11 is a front cross-sectional view of an asymmetric short n-channelIGFET configured according to the invention so as to have asemiconductor well portion of the same conductivity type as directlyunderlying semiconductor material.

FIGS. 12 a-12 c are respective graphs of individual, absolute, and netdopant concentrations as a function of longitudinal distance along theupper semiconductor surface for the IGFET of FIG. 11.

FIG. 13 is a front cross-sectional view of another asymmetric longn-channel IGFET configured according to the invention so as to have asemiconductor well portion of the same conductivity type as directlyunderlying semiconductor material.

FIGS. 14 a-14 c are respective graphs of individual, absolute, and netdopant concentrations as a function of longitudinal distance along theupper semiconductor surface for the IGFET of FIG. 13, 15, 18 b, or 18 c.

FIG. 15 is front cross-sectional view of a further asymmetric longn-channel IGFET configured according to the invention so as to have asemiconductor well portion of the same conductivity type as directlyunderlying semiconductor material.

FIGS. 16 a-16 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical lineextending through the source of the IGFET of FIG. 15.

FIGS. 17 a-17 c are respective graphs of individual, absolute, and netdopant concentration as a function of depth along a vertical lineextending through the drain of the IGFET of FIG. 15 or 18 c.

FIGS. 18 a-18 c are front cross-sectional views of three respective longn-channel IGFETs configured according to the invention so as to eachhave a semiconductor well portion of the same conductivity type asdirectly underlying semiconductor material.

FIGS. 19 a-19 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical lineextending through the source of the IGFET of FIG. 18 a or 18 b.

FIGS. 20 a-20 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical lineextending through the source of the IGFET of FIG. 18 c.

FIG. 21 is a front cross-sectional view of an asymmetric long n-channelIGFET configured according to the invention so as to have asemiconductor well portion of opposite conductivity type to directlyunderlying semiconductor material.

FIGS. 22 a-22 c are respective graphs of individual, absolute, and netdopant concentrations as a function of longitudinal distance along theupper semiconductor surface for the IGFET of FIG. 21 or 27 a.

FIGS. 23 a-23 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical linethrough the source of the IGFET of FIG. 21 or 25.

FIGS. 24 a-24 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical linethrough the drain of the IGFET of FIG. 21, 25, 27 a, or 27 b.

FIG. 25 is a front cross-sectional view of another asymmetric longn-channel IGFET configured according to the invention so as to have asemiconductor well portion of opposite conductivity type to directlyunderlying semiconductor material.

FIGS. 26 a-26 c are respective graphs of individual, absolute, and netdopant concentrations as a function of longitudinal distance along theupper semiconductor surface for the IGFET of FIG. 25 or 27 b.

FIGS. 27 a and 27 b are front cross-sectional views of two respectivelong n-channel IGFETs configured according to the invention so as toeach have a semiconductor well portion of opposite conductivity type todirectly underlying semiconductor material.

FIGS. 28 a-28 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical lineextending through the source of the IGFET of FIG. 27 a or 27 b.

FIGS. 29.1 and 29.2 are front cross-sectional views of two portions of acomplementary-IGFET semiconductor structure configured according to theinvention.

FIGS. 30.1 and 30.2 are front cross-sectional views of two portions ofanother complementary-IGFET semiconductor structure configured accordingto the invention.

FIGS. 31 a-31 o, 31 p.1-31 r.1, and 31 p.2-31 r.2 are frontcross-sectional views representing steps in manufacturing thecomplementary-IGFET semiconductor structure of FIGS. 29.1 and 29.2. Thesteps of FIGS. 31 a-31 o apply to the structural portions illustrated inboth of FIGS. 29.1 and 29.2. FIGS. 31 p.1-31 r.1 present further stepsleading to the structural portion of FIG. 29.1. FIGS. 31 p.2-31 r.2present further steps leading to the structural portion of FIG. 29.2.

FIGS. 32 a-32 c are front cross-sectional views representing steps of analternative, in accordance with the invention, to the step of FIG. 31 ein manufacturing a variation of the complementary-IGFET semiconductorstructure of FIGS. 29.1 and 29.2 starting with the structure of FIG. 31d repeated as FIG. 32 a.

FIGS. 33 a-33 f are front cross-sectional views representing steps ofanother alternative, in accordance with the invention, to the steps ofFIGS. 31 c-31 f in manufacturing a variation of the complementary-IGFETsemiconductor structure of FIGS. 29.1 and 29.2 starting with thestructure of FIG. 31 b repeated as FIG. 33 a.

FIG. 34 is a front cross-sectional view of an asymmetric long p-channelIGFET configured according to the invention so as to have asemiconductor well portion of opposite conductivity type to directlyunderlying semiconductor material and fabricated according to theinvention without using a compensatory n-type dopant implantation intosemiconductor material above the well portion as initially defined. Theasymmetric p-channel IGFET fabricated according to the process of FIGS.31 a-31 o, 31 p.1-31 r.1, and 31 p.2-31 r.2 using the alternative stepsof FIGS. 32 a-32 c or FIGS. 33 a-33 f is an implementation of thep-channel IGFET of FIG. 34.

FIGS. 35 a-35 c are respective graphs of individual, absolute, and netdopant concentrations as a function of longitudinal distance along theupper semiconductor surface for the IGFET of FIG. 34.

FIGS. 36 a-36 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical linethrough the source of the IGFET of FIG. 34.

FIGS. 37 a-37 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a pair of verticallines through the channel zone of the IGFET of FIG. 34.

FIGS. 38 a-38 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical linethrough the drain of the IGFET of FIG. 34.

FIGS. 39 and 40 are three-dimensional graphs of net dopant concentrationas a function of depth and longitudinal distance for respective computersimulations of (i) an asymmetric short re-channel IGFET configuredaccording to the invention and (ii) a reference symmetric shortn-channel IGFET.

FIGS. 41 and 42 are graphs presenting dopant contours as a function ofdepth and longitudinal distance from a source location for therespective computer-simulated IGFETs of FIGS. 39 and 40.

FIG. 43 is a graph of net dopant concentration as a function oflongitudinal distance from a source location for the computer-simulatedIGFETs of FIGS. 39 and 40.

FIGS. 44 a and 44 b are respective graphs of absolute and net dopantconcentrations as a function of depth along a pair of vertical linesrespectively through the source and drain for the computer-simulatedIGFETs of FIGS. 39 and 40.

FIGS. 45 a and 45 b are graphs of lineal transconductance and linealdrain current as a function of gate-to-source voltage respectively atthreshold and saturation conditions for the computer-simulated IGFETs ofFIGS. 39 and 40.

FIGS. 46 a and 46 b are graphs of lineal transconductance and linealdrain current as a function of gate-to-source voltage respectively atthreshold and saturation conditions for computer simulations of (i) aninventive asymmetric long n-channel IGFET generally corresponding to theinventive short-channel IGFET of FIG. 39 and (ii) a reference symmetriclong n-channel IGFET generally corresponding to the referenceshort-channel IGFET of FIG. 40.

FIG. 47 is a graph of lineal drain current density as a function ofgate-to-source voltage for computer simulations of (i) the inventiveIGFET of FIG. 39, (ii) the reference IGFET of FIG. 40, and (iii) afurther reference symmetric short n-channel IGFET lacking ananti-punchthrough implant.

FIG. 48 is a graph of lineal drain current as a function ofdrain-to-source voltage for the computer-simulated IGFETs of FIGS. 39and 40.

FIG. 49 is a circuit diagram of an n-channel IGFET and associatedparasitic capacitances.

FIG. 50 is a circuit diagram of a small-signal model of the n-channelIGFET and associated parasitic capacitances of FIG. 49.

FIGS. 51 a-51 c are circuit diagrams of single-IGFET amplifiers arrangedrespectively in common-source, common-gate, and common-drainconfigurations.

FIG. 52 is a circuit diagram of a single-IGFET amplifier arranged in acommon-source shorted-output configuration.

FIG. 53 is a circuit diagram of a small-signal model of the amplifier ofFIG. 52.

FIG. 54 is a graph of net dopant concentration as a function of distancefrom a pn junction for models of three different p-type dopantdistributions.

FIG. 55 is a graph of depletion-layer capacitance as a function ofreverse voltage for the models of the three dopant distributions of FIG.54.

FIG. 56 is a graph of net body dopant concentration as a function ofdistance from a pn junction for a model of a junction capacitor whosemore lightly doped side has a dopant profile that undergoes a stepchange in dopant concentration.

FIG. 57 is graph of areal junction capacitance as a function of reversevoltage for the junction capacitor modeled in FIG. 56.

FIGS. 58 a and 58 b are composite front cross-sectional views/graphs ofdopant contours as a function of depth and longitudinal distance fromthe channel center for computer simulations of respective asymmetricshort and long n-channel IGFETs configured according to the invention.

FIG. 59 is a graph of lineal drain-to-body capacitance as a function ofdrain-to-body voltage for the computer-simulated IGFETs of FIGS. 39 and40.

FIG. 60 is a graph of lineal source-to-body capacitance as a function ofsource-to-body voltage for the computer-simulated IGFETs of FIGS. 39 and40.

FIG. 61 is a graph of cut-off frequency as a function of lineal draincurrent for the computer-simulated IGFETs of FIGS. 39 and 40 and thefurther inventive IGFET of FIG. 63.

FIG. 62 is a graph of cut-off frequency as a function of lineal draincurrent for computer simulations of (i) an inventive asymmetric longn-channel IGFET corresponding to the inventive short-channel IGFET ofFIG. 39, (ii) a reference symmetric long n-channel IGFET correspondingto the reference short-channel IGFET of FIG. 40, and (iii) a furtherinventive asymmetric long re-channel IGFET corresponding to the furtherinventive short-channel IGFET of FIG. 63.

FIG. 63 is a front cross-sectional view of another computer-simulatedasymmetric short re-channel IGFET configured according to the invention.

FIG. 64 is a graph of net dopant concentration as a function oflongitudinal distance from a source location for the computer-simulatedIGFET of FIG. 63.

FIG. 65 is a graph of threshold voltage as a function of channel lengthfor (i) asymmetric n-channel IGFETs configured according to theinvention, (ii) reference symmetric n-channel IGFETs having halo pocketportions along each source/drain zone, and (iii) reference symmetricre-channel IGFETs lacking a halo pocket along each source/drain zone.

FIG. 66 is a front cross-sectional view of an additionalcomplementary-IGFET semiconductor structure configured according to theinvention.

FIG. 67 is a graph of absolute dopant concentration as a function ofdepth for (i) two asymmetric n-channel IGFETs configured according tothe invention and (ii) a reference symmetric n-channel IGFET.

FIGS. 68 a and 68 b are front cross-sectional views of two respectivefurther asymmetric long n-channel IGFETs configured according to theinvention.

FIGS. 69 a-69 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical linethrough the source of the IGFET of FIG. 68 a or 68 b.

FIGS. 70 a-70 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a pair of verticallines through the channel zone of the IGFET of FIG. 68 a or 68 b.

FIGS. 71 a-71 c are respective graphs of individual, absolute, and netdopant concentrations as a function of depth along a vertical lineextending through the drain of the IGFET of FIG. 68 a or 68 b.

FIGS. 72 a-72 d are front cross-sectional views of four additionalrespective complementary-IGFET semiconductor structures configuredaccording to the invention.

Like reference symbols are employed in the drawings and in thedescription of the preferred embodiment to represent the same, or verysimilar, item or items. The numerical portions of reference symbolshaving single prime (′), double prime (″), asterisk (*), and pound(^(#)) signs in drawings containing graphs respectively indicatelike-numbered regions or zones in other drawings. The “Xs” in across-sectional view of an IGFET provided with a well dopant indicatethe location of the maximum concentration of the well dopant.Electrically insulating spacers (not shown) may be situated along thesidewalls of the gate electrodes of the IGFETs of FIGS. 13, 15, 18 b, 18c, 25, 27 b, and 34 depending on how those IGFETs are fabricated.

In the dopant-distribution graphs, “individual” dopant concentrationsmean the individual concentrations of each separately introduced n-typedopant and each separately introduced p-type dopant while “absolute”dopant concentrations mean the total n-type dopant concentration and thetotal p-type dopant concentration. The “net” dopant concentration in thedopant-distribution graphs is the difference between the absolute (ortotal) n-type dopant concentration and the absolute (or total) p-typedopant concentration. The net dopant concentration is indicated as net“n-type” where the absolute n-type dopant concentration exceeds theabsolute p-type dopant concentration, and as net “p-type” where theabsolute p-type dopant concentration exceeds the absolute n-type dopantconcentration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference Notation and OtherConventions

The reference symbols employed below and in the drawings have thefollowing meanings where the adjective “lineal” means per unit IGFETwidth and where the adjective “areal” means per unit lateral area:

-   A_(I)≡current gain-   C_(da)≡areal depletion-region capacitance-   C_(d0a)≡value of areal depletion-region capacitance at zero reverse    voltage-   C_(DB)≡drain-to-body capacitance-   C_(DBw)≡lineal drain-to-body capacitance-   C_(GB)≡gate-to-body capacitance-   C_(GD)≡gate-to-drain capacitance-   C_(GIa)≡areal gate dielectric capacitance-   C_(GS)≡gate-to-source capacitance-   C_(L)≡load capacitance-   C_(SB)≡source-to-body capacitance-   C_(SBw)≡lineal source-to-body capacitance-   f≡frequency-   f_(T)≡cut-off frequency-   f_(Tpeak)≡peak value of cut-off frequency-   g_(m)≡intrinsic transconductance of IGFET-   g_(mw)≡lineal transconductance of IGFET-   g_(mb)≡transconductance of body electrode-   g_(meff)≡effective transconductance of IGFET in presence of source    resistance-   g_(msatw)≡lineal transconductance of IGFET at saturation-   H_(A)≡amplifier transfer function-   I_(D)≡drain current-   I_(Dw)≡lineal drain current-   I_(D0w)≡leakage value of lineal drain current at zero gate-to-source    voltage-   i_(i)≡small-signal input current-   i_(o)≡small-signal output current-   K_(S)≡relatively permittivity of semiconductor material-   k≡Boltzmann's constant-   L≡length of channel-   L_(G)≡length of gate electrode-   L_(GDoverlap)≡longitudinal distance that gate electrode overlies    drain-   L_(GSoverlap)≡longitudinal distance that gate electrode overlaps    source-   N_(A)≡acceptor dopant concentration-   N_(B)≡net dopant concentration in body material-   N_(B0), N_(B0)′≡values of net dopant concentration in    junction-adjoining constant-concentration portion of material on    more lightly doped side of pn junction-   N_(B1), N_(B1)′≡values of net dopant concentration in    junction-remote constant-concentration portion of material on more    lightly doped side of pn junction-   N_(D)≡donor dopant concentration-   N_(D0)≡value of net dopant concentration in constant-concentration    material on more heavily doped side of pn junction-   N_(I)≡individual dopant concentration-   N_(N)≡net dopant concentration-   N_(T)≡absolute dopant concentration-   n_(i)≡intrinsic carrier concentration-   q≡electronic charge-   R_(D)≡series resistance at drain of IGFET-   R_(G)≡series resistance at gate electrode of IGFET-   R_(on)≡linear-region on-resistance of IGFET-   R_(S)≡series resistance at source of IGFET-   s≡transform variable-   T≡temperature-   t_(d)≡depletion-region thickness-   t_(d0)≡value of depletion-region thickness at zero reverse voltage-   t_(GI)≡gate dielectric thickness-   V_(BI)≡built-in voltage-   V_(BS)≡DC body-to-source voltage-   V_(DB)≡DC drain-to-body voltage-   V_(DD)≡high supply voltage-   V_(DS)≡DC drain-to-source voltage-   V_(GS)≡DC gate-to-source voltage-   V_(g)≡gate voltage amplitude-   y_(in)≡input voltage amplitude-   V_(out)≡output voltage amplitude-   V_(R)≡DC reverse voltage-   V_(Rmax)≡maximum value of DC reverse voltage-   V_(SB)≡DC source-to-body voltage-   V_(SS)≡low supply voltage-   V_(T)≡threshold voltage-   V_(gs)≡small-signal gate-to-source voltage-   V_(nsat)≡electron saturation velocity-   W≡channel width-   x≡longitudinal distance-   y≡depth, vertical distance, or distance from pn junction-   y_(d)≡value of distance from pn junction to remote boundary of    depletion region in body material-   y_(d0)≡value of distance from pn junction to remote boundary of    junction-adjoining constant-concentration portion of body material    having step change in uniform net dopant concentration-   y_(dmax)≡value of distance from pn junction to remote boundary of    junction-remote constant-concentration portion of body material    having step change in uniform net dopant concentration-   y_(D)≡value of depth at bottom of drain-   y_(S)≡value of depth at bottom of source-   y_(ST)≡value of depth at remote boundary of junction-adjoining upper    body-material portion-   y_(W)≡value of depth at location of maximum concentration of well    dopant-   ∈₀≡permittivity of free space (vacuum)-   μ_(n)≡electron mobility-   ω≡angular frequency-   ω_(in)≡value of angular frequency at input pole-   ω_(out)≡value of angular frequency at output pole-   ω_(z)≡value of angular frequency at zero-   ω_(p)≡value of angular frequency at pole

Long-channel and short-channel n-channel IGFETs are respectivelyreferred to here, i.e., both below and above, as long and shortn-channel IGFETs. Similarly, long-channel and short-channel p-channelIGFETs are respectively referred to here as long and short p-channelIGFETs. As used below, the term “surface-adjoining” means adjoining (orextending to) the upper semiconductor surface, i.e., the upper surfaceof a semiconductor body consisting of monocrystalline, or largelymonocrystalline, semiconductor material.

No particular channel-length value generally separates the short-channeland long-channel regimes of IGFET operation or generally distinguishes ashort-channel IGFET from a long-channel IGFET. A short-channel IGFET, oran IGFET operating in the short-channel regime, is simply an IGFET whosecharacteristics are significantly affected by short-channel effects. Along-channel IGFET, or an IGFET operating in the long-channel regime, isthe converse of a short-channel IGFET. While the channel length value ofapproximately 0.4 mm roughly constitutes the boundary between theshort-channel and long-channel regimes for the background art in U.S.Pat. No. 6,548,642 B1, the long-channel/short-channel boundary can occurat a higher or lower value of channel length depending on variousfactors such as gate dielectric thickness, minimum printable featuresize, channel zone dopant concentration, and source/drain-body junctiondepth. IGFETs in which Vertical Body-Material Dopant Profile Below Drainis Hypoabrupt Due to Subsurface Maximum in Well Dopant Concentration

FIG. 6 illustrates an asymmetric long n-channel IGFET 100 configured inaccordance with the invention so as to be especially suitable forhigh-speed analog applications. Long-channel IGFET 100 is created from amonocrystalline silicon (“monosilicon”) semiconductor body in which apair of very heavily doped n-type source/drain (again, “S/D”) zones 102and 104 are situated along the upper semiconductor surface. S/D zones102 and 104 are generally referred to below respectively as source 102and drain 104 because they normally, though not necessarily,respectively function as source and drain.

Drain 104 is normally doped slightly more heavily than source 102. Themaximum value of net dopant concentration N_(N) in source 102 along theupper semiconductor surface is normally at least 1×10²⁰ atoms/cm³,typically 4×10²⁰ atoms/cm³. The maximum value of concentration N_(N) indrain 104 along the upper surface is normally at least 1×10²⁰ atoms/cm³,typically slightly greater than 4×10²⁰ atoms/cm³ so as to slightlyexceed the maximum upper-surface N_(N) concentration in source 102.However, as discussed below in connection with the inventive IGFET ofFIG. 63, drain 104 is sometimes doped more lightly than source 102. Forexample, the maximum value of concentration N_(N) in drain 104 along theupper surface can be 5×10¹⁹ atoms/cm³ and can go down to at least aslittle as 1×10¹⁹ atoms/cm³ when maximum upper-surface N_(N)concentration in source 102 is at least 1×10²⁰ atoms/cm³.

Source 102 extends to a distance y_(S) below the upper semiconductorsurface. Drain 104 extends to a depth y_(D) below the uppersemiconductor surface. Source depth y_(S) is normally 0.1-0.2 μm,typically 0.15 μm. Drain depth y_(D) is normally 0.15-0.3 μm, typically0.2 μm. Drain depth y_(D) thus normally exceeds source depth y_(S),typically by 0.05-0.1 μm.

Source 102 and drain 104 are laterally separated by an asymmetricchannel zone 106 of p-type body material 108 that forms (a) asource-body pn junction 110 with source 102 and (b) a drain-body pnjunction 112 with drain 104. P-type body material 108 consists of alightly doped lower portion 114, a heavily doped intermediate wellportion 116, and an upper portion 118 that typically extends deeperbelow the upper semiconductor surface than source 102 and drain 104.Upper body-material portion 118 thereby typically contains all ofchannel zone 106. In any event, p-lower body-material portion 114 and p+well portion 116 extend laterally below Source 102 and drain 104.

P+ well portion 116 is defined by p-type semiconductor well dopantdistributed vertically in a roughly Gaussian manner so as to reach amaximum subsurface concentration at a depth y_(W) below the uppersemiconductor surface. The “Xs” in FIG. 6 generally indicate thelocation of the maximum subsurface concentration of the p-type welldopant. The concentration of the p-type well dopant at depth y_(W) isnormally 1×10¹⁸-1×10¹⁹ atoms/cm³, typically 5×10¹⁸ atoms/cm³. Maximumwell-concentration depth y_(W), which exceeds source depth y_(S) anddrain depth y_(D), is normally 0.5-1.0 μm, typically 0.7 μm.Additionally, depth y_(W) is normally no more than 10 times, preferablyno more than 5 times, drain depth y_(D). That is, the location of themaximum concentration of the p-type well dopant is no more than 10times, preferably no more than 5 times, deeper below the upper surfacethan drain 104.

The upper and lower boundaries of heavily doped well portion 116 aresomewhat imprecise because well 116 is situated in doped semiconductormaterial of the same conductivity (p type) as well 116. Thesemiconductor material that bounds well 116 does, as indicated below,have a low p-type background dopant concentration which is normallyrelatively uniform. The upper and lower boundaries of well 116 aretypically defined as the locations where the concentration of the p-typewell dopant equals the p-type background dopant concentration. Asidefrom any location where well 116 extends into other p-type materialdoped more heavily than well 116, the concentration of the total p-typedopant along the upper and lower boundaries of well 116 is then twicethe p-type background dopant concentration. Under these boundarydefinitions, the upper boundary of well 116 is normally 0.2-0.5 μm,typically 0.3 μm, below the upper semiconductor surface. The lowerboundary of well 116 is normally 0.9-1.3 μm, typically 1.1 μm, below theupper surface.

A depletion region (not shown) extends along the upper semiconductorsurface from source-body pn junction 110 across channel zone 106 todrain-body pn junction 112 during IGFET operation. The average thicknessof the surface depletion region is normally less than 0.1 μm, typicallyin the vicinity of 0.05 μm. Although the upper and lower boundaries ofwell portion 116 are somewhat imprecise, the concentration of the p-typewell dopant normally drops to an electrically insignificant level at adepth less than 0.1 μm below the upper surface. Accordingly, well 116 issubstantially located below the surface depletion region.

P-type upper body-material portion 118 includes a heavily doped pocketportion 120 that extends along source 102 up to the upper semiconductorsurface and terminates at a location between source 102 and drain 104.FIG. 6 illustrates the example in which p+ pocket portion 120 extendsdeeper below the upper surface than source 102 and drain 104. Inparticular, FIG. 6 depicts the example in which pocket portion 120extends laterally below source 102 and largely reaches p+ well portion116. As discussed below in connection with FIGS. 18 a-18 c, pocketportion 120 can extend to a lesser depth below the upper surface thanshown in FIG. 6. The remainder of p-type upper body-material portion118, i.e., the part outside pocket portion 120, is indicated as item 124in FIG. 6. Upper body-material remainder 124 is lightly doped andextends along drain 104. Channel zone 106, which consists of all thep-type semiconductor material between source 102 and drain 104, isthereby formed by part of source-side p+ pocket portion 120 and part ofdrain-side p− upper body-material remainder 124.

A gate dielectric layer 126 is situated on the upper semiconductorsurface and extends over channel zone 106. A gate electrode 128 issituated on gate dielectric layer 126 above channel zone 106. Gateelectrode 128 extends partially over source 102 and drain 104. In theexample of FIG. 6, gate electrode 128 consists of polycrystallinesilicon (“polysilicon”) doped very heavily n type. Gate electrode 128can be formed with other electrically conductive material such as metalor polysilicon doped sufficiently p type as to be electricallyconductive.

The upper surfaces of source 102, drain 104, and n++ gate electrode 128are typically respectively provided with thin layers (not shown in FIG.6) of electrically conductive metal silicide to facilitate makingelectrical contact to regions 102, 104, and 128. In that case, gateelectrode 128 and the overlying metal silicide layer form a compositegate electrode. Source 102, drain 104, and channel zone 106 aretypically laterally surrounded by an electrical insulating field region(likewise not shown in FIG. 6) recessed into the upper semiconductorsurface to define an active semiconductor island that contains regions102, 104, and 106. Examples of the metal silicide layers and thefield-insulating region are presented blow in connection with FIGS. 29.1and 29.2.

The presence of p+ pocket portion 120 along source 102 causes channelzone 106 to be graded longitudinally, i.e., in the direction of thechannel length, with respect to channel dopant concentration. Because asubstantial mirror image of source-side pocket portion 120 is notsituated along drain 104, channel zone 106 is asymmetrically dopantgraded in the longitudinal direction. P+ well portion 116 is situatedbelow p− upper body-material remainder 124 that extends along drain 104.This configuration of p+ well 116 and p− upper body-material remainder124 causes the vertical dopant profile in the portion of body material108 underlying drain 102 to be hypoabrupt. That is, the concentration ofthe p-type dopant increases greatly, normally by at least a factor of10, in going from drain-body junction 112 downward through p− upperbody-material remainder 124 and into p+ well 116. The combination of thelongitudinally asymmetric dopant grading in channel zone 106 and thehypoabrupt vertical dopant profile through drain 104 in the portion ofbody material 108 underlying drain 104 enables IGFET 100 to have verygood analog characteristics while avoiding punchthrough.

An understanding of the longitudinally asymmetric dopant grading inchannel zone 106 and the hypoabrupt vertical dopant profile in theportion of body material 108 underlying drain 104 is facilitated withthe assistance of FIGS. 7 a-7 c (collectively “FIG. 7”), FIGS. 8 a-8 c(collectively “FIG. 8”), FIGS. 9 a-9 c (collectively “FIG. 9”), andFIGS. 10 a-10 c (collectively “FIG. 10”). FIG. 7 presents exemplarydopant concentrations along the upper semiconductor surface as afunction of longitudinal distance x. Exemplary dopant concentrations asa function of depth y along a vertical line 130 through source 102 arepresented in FIG. 8. FIG. 9 presents exemplary dopant concentrations asa function of depth y along a pair of vertical lines 132 and 134 throughchannel zone 106. Vertical line 132 passes through source-side pocketportion 120. Vertical line 134 passes through a vertical locationbetween pocket portion 120 and drain 104. Exemplary dopantconcentrations as a function of depth y along a vertical line 136through drain 104 are presented in FIG. 10.

FIG. 7 a specifically illustrates concentrations N_(I), along the uppersemiconductor surface, of the individual semiconductor dopants thatlargely define regions 102, 104, 120, and 124 and thus establish thelongitudinal dopant grading of channel zone 106. FIGS. 8 a, 9 a, and 10a specifically illustrate concentration N_(I), along vertical lines 130,132, 134, and 136, of the individual semiconductor dopants thatvertically define regions 102, 104, 114, 116, 120, and 124 and thusestablish the hypoabrupt vertical dopant profile in the portion of bodymaterial 108 underlying drain 104. Curves 102′ and 104′ representconcentrations N_(I) (surface and vertical) of the n-type dopant used torespectively form source 102 and drain 104. Curves 114′, 116′, 120′, and124′ represent concentrations N_(I) (surface and/or vertical,) of thep-type dopants used to respectively form regions 114, 116, 120, and 124.Items 110 ^(#) and 112 ^(#) indicate where net dopant concentrationN_(N) goes to zero and thus respectively indicate the locations of pnjunctions 110 and 112.

Concentrations N_(T) of the total p-type and total n-type dopants inregions 102, 104, 120, and 124 along the upper semiconductor surface areshown in FIG. 7 b. FIGS. 8 b, 9 b, and 10 b depict, along vertical lines130, 132, 134, and 136, concentrations N_(T) of the total p-type andtotal n-type dopants in regions 102, 104, 114, 116, 120, and 124. Curvesegments 114″, 116″, 120″, and 124″ respectively corresponding toregions 114, 116, 120, and 124 represent total concentrations N_(T) ofthe p-type dopant. Item 106″ in FIG. 7 b corresponds to channel zone 106and represents the channel-zone portions of curve segments 120″ and124″. Total concentrations N_(T) of the n-type dopant are represented bycurves 102″ and 104″ respectively corresponding to source 102 and drain104. Curves 102″ and 104″ in FIG. 7 b are respectively identical tocurves 102′ and 104′ in FIG. 7 a. Curves 102″ and 104″ in FIGS. 8 b and10 b are respectively identical to curves 102′ and 104′ in FIGS. 8 a and10 a.

FIG. 7 c illustrates net dopant concentration N_(N) along the uppersemiconductor surface. Net dopant concentration N_(N) along verticallines 130, 132, 134, and 136 is presented in FIGS. 8 c, 9 c, and 10 c.Curve segments 114*, 116*, 120*, and 124* represent net concentrationsN_(N) of the p-type dopant in respective regions 114, 116, 120, and 124.Item 106* in FIG. 7 c represents the combination of channel-zone curvesegments 120* and 124* and thus presents concentration N_(N) of the netp-type dopant in channel zone 106. Concentrations N_(N) of the netn-type dopant in source 102 and drain 104 are respectively representedby curves 102* and 104*.

With the foregoing general comments about FIGS. 7-10 in mind, FIG. 7 aindicates that the p-type dopant in source-side pocket portion 120 hastwo primary components, i.e., components provided in two separate dopingoperations, along the upper semiconductor surface. One of the primarycomponents of the p-type dopant in pocket portion 120 along the uppersurface is the p-type background dopant represented by curve 124′ inFIG. 7 a. The p-type background dopant is normally present at a low,largely uniform, concentration throughout all of the monosiliconmaterial including regions 102, 104, 114, 116, and 120. Below pocket 120and upper body-material remainder 124, the p-type background dopant isrepresented by curve segment 114′ as indicated in FIGS. 8 a, 9 a, and 10a. The concentration of the p-type background dopant is normally1×10¹⁵-1×10¹⁶ atoms/cm³, typically 5×10¹⁵ atoms/cm³.

The other primary component of the p-type dopant in source-side pocketportion 120 is the p-type pocket (or channel-grading) dopant indicatedby curve 120′ in FIG. 7 a. The p-type pocket dopant is provided at ahigh upper-surface concentration, normally 5×10¹⁷-2×10¹⁸ atoms/cm³,typically 1×10¹⁸ atoms/cm³, to define pocket portion 120. The specificvalue of the upper-surface concentration of the p-type pocket dopant iscritically adjusted, typically within 10% accuracy, to set the thresholdvoltage of IGFET 100.

The boundary of source-side pocket portion 120 consists of (a) a sectionof the upper semiconductor surface, (b) a pn junction section formed bysource-body junction 110, and (c) a p-type section of body material 108.Although the p-type section of the boundary of pocket 120 is somewhatimprecise, the p-type pocket section is typically defined as thelocation where the concentration of the p-type pocket dopant equals theconcentration of the p-type background dopant. To the extent that pocket120 does not intrude into well portion 116, the p-type dopantconcentration along the p-type section of the boundary of pocket 120 isthen twice the background dopant concentration, including where thep-type pocket-portion boundary section meets the upper semiconductorsurface.

The p-type pocket dopant is also present in source 102 as indicated bycurve 120′ in FIG. 7 a. Concentration N_(I) of the p-type pocket dopantin source 102 is substantially constant along its upper surface. Inmoving from source 102 longitudinally along the upper semiconductorsurface into channel zone 106, concentration N_(I) of the p-type pocketdopant is at the substantially constant upper-surface source levelpartway into zone 106 and then drops from that level essentially to zeroat a location between source 102 and drain 104.

With the total p-type dopant in channel zone 106 along the uppersemiconductor surface being the sum of the p-type background and pocketdopants along the upper surface, the total p-type channel-zone dopantalong the upper surface is represented by curve segment 106″ in FIG. 7b. The variation in curve segment 106″ shows that, in movinglongitudinally across channel zone 106 from source 102 to drain 104,concentration N_(T) of the total p-type dopant in zone 106 along theupper surface is at a substantially constant high level partway intozone 106, decreases from the high level to the low p-type backgroundlevel at a location between source 102 and drain 104, and then remainsat the low background level for the rest of the distance to drain 104.

Concentration N_(I) of the p-type pocket dopant in source 102 may, insome embodiments, be at the substantially constant source level alongonly part of the upper surface of source 102 and may then decrease inmoving longitudinally along the upper semiconductor surface from alocation within the upper surface of source 102 to source-body junction110. In that case, concentration N_(I) of the p-type pocket dopant inchannel zone 106 starts decreasing immediately after crossingsource-body junction 110 in moving longitudinally across zone 106 towarddrain 104. Accordingly, concentration N_(T) of the total p-type dopantin zone 106 along the upper surface similarly starts to decreaseimmediately after crossing junction 110 in moving longitudinally acrosschannel zone 106 from source 102 to drain 104 rather than being at thesubstantially constant source level partway into zone 106.

Regardless of whether concentration N_(I) of the p-type pocket dopant inchannel zone 106 along the upper semiconductor surface is, or is not, atthe substantially source level for a non-zero distance from source-bodyjunction 110 longitudinally into zone 106, concentration N_(T) of thetotal p-type dopant in zone 106 along the upper surface is lower wherezone 106 meets drain 104 than where zone meets 106 source 102. Inparticular, concentration N_(T) of the total p-type dopant in zone 106is normally at least a factor of 10 lower, preferably at least a factorof 20 lower, more preferably at least a factor of 50 lower, typically afactor of 100 or more lower, at drain-body junction 112 along the uppersurface than at source-body junction 110 along the upper surface.

FIG. 7 c shows that, as represented by curve 106*, concentration N_(N)of the net p-type dopant in channel zone 106 along the uppersemiconductor surface varies in a similar manner to concentration N_(T)of the total p-type dopant in zone 106 along the upper surface exceptthat net dopant concentration N_(N) of the net p-type dopant in zone 106along the upper surface drops to zero at pn junctions 110 and 112. Thesource side of channel zone 106 thus has a high net amount of p-typedopant compared to the drain side. The high source-side amount of p-typedopant in channel zone 106 causes the thickness of the channel-sideportion of the depletion region along source-body junction 110 to bereduced.

Also, the high p-type dopant concentration along the source side ofchannel zone 106 shields source 102 from the comparatively high electricfield in drain 104. This occurs because the electric field lines fromdrain 104 terminate on ionized p-type dopant atoms in pocket portion 120instead of terminating on ionized dopant atoms in the depletion regionalong source 102 and detrimentally lowering the potential barrier forelectrons. The depletion region along source-body junction 110 isthereby inhibited from punching through to the depletion region alongdrain-body junction 112. By appropriately choosing the amount of thehigh source-side p-type dopant in channel zone 106, punchthrough isavoided in IGFET 100.

The p-type dopant in the portion of body material 108 below source 102,channel zone 106, and drain 104 has three primary components asindicated in FIGS. 8 a, 9 a, and 10 a. One of the primary components ofthe p-type dopant in the body-material portion below regions 102, 104,and 106 is the p-type background dopant represented by curve segment124′ or 114′ in FIGS. 8 a, 9 a, and 10 a. The second primary componentis the p-type well dopant that defines well portion 116 as indicated bycurve 116′ in FIGS. 8 a, 9 a, and 10 a.

The final primary component of the p-type dopant in the body-materialportion below regions 102, 104, and 106 is the p-type pocket dopantindicated by curve 120′ in FIGS. 8 a and 9 a. The p-type pocket dopantis present only in the portion of body material 108 below source 102 andan adjoining part of channel zone 106. The amount of p-type pocketdopant present in the portion of body material 108 below drain 104 iseither essentially zero or is so low as to be substantially electricallyinsignificant. Hence, the total p-type pocket dopant in the portion ofbody material 108 below drain 104 consists substantially only of thep-type well and background dopants as indicated respectively by curves116′ and 124′ or 114′ taken along vertical line 136 in FIG. 10 a.

The total p-type dopant in the portion of body material 108 below drain104 is indicated by curve segment 116″ and its extensions 124″ (upward)and 114″ (downward) in FIG. 10 b. Curve segment 116″ represents the sumof the p-type well and background dopants in well portion 116. Curvesegments 114″ and 124″ respectively correspond to p− lower body-materialportion 114 and p-upper body-material remainder 124. Becauseconcentration N_(I) of the p-type background dopant is relativelyuniform, concentration N_(T) of the total p-type dopant in the portionof body material 108 below drain 104 reaches a maximum value at asubsurface location substantially equal to depth y_(W), i.e.,substantially where the p-type well dopant reaches its maximumconcentration.

As shown by the variation in combined curve segment 116″/124″ in FIG. 10b, concentration N_(T) of the total p-type dopant in the portion of bodymaterial 108 below drain 104 decreases hypoabruptly by at least a factorof 10 in moving upward along vertical line 136 to drain 104 from thesubsurface location of the maximum concentration of the p-type dopant inwell portion 116. Concentration N_(T) of the total p-type dopant in theportion of body material 108 below drain 104 preferably decreases by atleast a factor of 20, more preferably by at least a factor of 40, evenmore preferably by at least a factor of 80, typically by a factor in thevicinity of 100 or more, in moving from the location of the maximump-type well concentration up to drain 104. Additionally, concentrationN_(T) of the total p-type dopant in the portion of body material 108below drain 104 normally decreases progressively in moving from thelocation of the maximum p-type well concentration up to drain 104 asindicated by combined curve segment 116″/124″.

The net dopant in the portion of body material 108 below drain 104 isp-type dopant. FIG. 10 c shows that, as represented by the combinationof curve segments 116* and 124*, concentration N_(N) of the net dopantin the portion of body material 108 below drain 104 varies vertically ina similar manner to concentration N_(T) of the total p-type dopant inthe portion of body material 108 below drain 104 except thatconcentration N_(N) in the portion of body material 108 below drain 104drops to zero at drain-body junction 112. The hypoabrupt vertical dopantprofile in the portion of body material 108 below drain 104 causes theparasitic capacitance associated with drain-body junction 112 todecrease for the reasons discussed further below. This enables IGFET 100to have increased analog speed.

Moving to the dopant distribution along vertical line 130 through source102, the total p-type dopant in the portion of body material 108 belowsource 102 consists of the p-type well, background, and pocket dopantsas indicated respectively by curves 116′, 124′, and 120′ in FIG. 8 a.For the example of FIG. 6 in which source-side pocket portion 120 meetswell 116 below source 102 so that concentrations N_(I) of the p-typewell and pocket dopants both exceed the p-type background concentrationat the meeting location, the total p-type dopant in the portion of bodymaterial 108 below source 102 is indicated by the combination of curvesegments 116″ and 120″ in FIG. 8 b. In an embodiment where pocket 120extends below drain 102 but does not meet well portion 116, the totalp-type dopant in the portion of body material 108 below source 102 wouldbe indicated by the combination of curve segments 116″ and 120″ and acurve segment corresponding to p− upper body-material remainder 124.

As illustrated by combined curve segment 116″/120″ in FIG. 8 b,concentration N_(T) of the total p-type dopant in the portion of bodymaterial 108 below source 102 initially decreases by a factor of 10,typically by a factor in the vicinity 30, in moving upward through bodymaterial 108 from the subsurface location of the maximum concentrationof the p-type dopant in well portion 116. Upon reaching a local minimumbelow source 102, concentration N_(T) of the total p-type dopant in theportion of body material 108 below source 102 then rises before reachingsource 102.

The net dopant in the portion of body material 108 below drain 102 isp-type dopant. FIG. 8 c shows that, as represented by the combination ofcurve segments 116* and 120* for the example of FIG. 6 in whichsource-side pocket portion 120 meets well portion 116, concentrationN_(N) of the net dopant in the portion of body material 108 below source102 varies vertically in a similar manner to concentration N_(T) of thetotal p-type dopant in the portion of body material 108 below source 102except that net dopant concentration N_(N) in the portion of bodymaterial 108 below source 102 drops to zero at source-body junction 110.Depending on various factors such as the amount by which the depth ofpocket 120 exceeds the depth of source 102, this vertical dopant profilebelow source 102 sometimes causes the parasitic capacitance alongsource-body junction 110 to be reduced, albeit normally less than thereduction in the parasitic capacitance along drain-body junction 112.

Turning briefly to FIG. 9 dealing with dopant concentrations N_(I),N_(T), and N_(N) along vertical lines 132 and 134 through channel zone106, the parenthetical entry “132” after reference symbols 120′, 120″,and 120* indicates dopant concentrations along vertical line 132.Parenthetical entry “134” after reference symbols 124′, 124″, and 124*indicates dopant concentrations along vertical line 134.

FIG. 11 depicts an asymmetric short n-channel IGFET 140 configured inaccordance with the invention so as to be particularly suitable forhigh-speed analog applications. Short-channel IGFET 140 is a variationof long-channel IGFET 100 in which the channel length is shortened tosuch an extent that IGFET operation occurs in the short-channel regime.The channel length is shortened by appropriately reducing the length ofgate electrode 128. In the example of FIG. 11, p-type pocket portion 120extends sufficiently far across channel zone 106 to just meet drain 104.

FIGS. 12 a-12 c (collectively “FIG. 12”) respectively present exemplarydopant concentrations N_(I), N_(T), and N_(N) along the uppersemiconductor surface of IGFET 140 as a function of longitudinaldistance x in order to facilitate understanding the asymmetriclongitudinal dopant grading in channel zone 106 of IGFET 140. All of theanalysis presented above for IGFET 100 in connection with FIG. 7 appliesto IGFET 140 except that the shortened length of channel zone 106 inIGFET 140 causes the dopant distribution in channel zone 106 of IGFET140 to differ from that in channel zone 106 of IGFET 100 as explainedbelow in connection with FIG. 12.

As in FIG. 7 a, concentration N_(I) of the p-type pocket dopant alongthe upper semiconductor surface is represented by curve 120′ in FIG. 12a. Unlike what occurs in channel zone 106 of IGFET 100, concentrationN_(I) of the p-type pocket dopant along the upper surface of channelzone 106 of IGFET 140 does not drop essentially to zero at a locationbetween source 102 and drain 104. Instead, as illustrated by curve 120′in FIG. 12 a, concentration N_(I) of the p-type pocket dopant along theupper surface of channel zone 106 of IGFET 140 is greater than zeroalong substantially all of the channel zone's upper surface and thus isat a small finite value where channel zone 106/pocket portion 120 meetsdrain 104. Concentration N_(I) of the pocket dopant in IGFET 140 is at amuch greater value where zone 106/pocket 120 meets source 102 along thechannel zone's upper surface.

The preceding change in concentration N_(I) of the p-type pocket dopantalong the upper surface of channel zone 106 in IGFET 140 is reflected inabsolute dopant concentration N_(T) and net dopant concentration N_(N)along the upper semiconductor surface of IGFET 140. As in FIG. 7 b,curve segment 106″ in FIG. 12 b represents concentration N_(T) of thetotal p-type dopant in channel zone 106 along the upper semiconductorsurface for which the total p-type dopant in zone 106 along the uppersurface is the sum of the p-type background and pocket dopants along theupper semiconductor surface. The variation of curve segment 106″ in FIG.12 b shows that, in moving from source 102 through channel zone 106 todrain 104 along the upper semiconductor surface, concentration N_(T) ofthe total p-type dopant in zone 106 along the upper semiconductorsurface of IGFET 140 is substantially constant at the high level insource 102 partway into zone 106 and then decreases from the high levelto a low level slightly greater than the background concentration uponreaching drain 104. More particularly, concentration N_(T) of the totalp-type dopant in channel zone 106 of IGFET 140 increases progressivelyin going along the upper semiconductor surface from where zone 106 meetsdrain 104 partway to where zone 106 meets source 102.

Similar to what was said above about IGFET 100, concentration N_(I) ofthe p-type pocket dopant along the upper semiconductor surface in IGFET140 may start decreasing immediately after passing source-body pnjunction 110 in moving longitudinally along the upper surface fromsource 102 across channel zone 106 to drain 104. As viewed in movingfrom drain 104 to source 102 rather than from source 102 to drain 104,concentration N_(T) of the total p-type dopant in channel zone 106 inIGFET 140 then increases progressively in going along the uppersemiconductor surface from where zone 106 meets drain 104 all the way towhere zone 106 meets source 102. In any event, concentration N_(T) ofthe total p-type dopant in zone 106 along the upper surface of IGFET 140satisfies the specifications presented above for IGFET 100 of beinglower where zone 106 meets drain 104 than where zone 106 meets source102.

FIG. 12 c shows that, as represented by curve 106*, concentration N_(N)of the net p-type dopant along the upper surface of channel zone 106 ofIGFET 140 varies in a similar manner to concentration N_(T) of the totalp-type dopant in zone 106 of IGFET 140 along the upper semiconductorsurface except that concentration N_(N) of the net p-type dopant in zone106 of IGFET 140 along the upper semiconductor surface goes to zero atpn junctions 110 and 112. As in channel zone 106 of IGFET 100, thesource side of channel zone 106 in IGFET 140 has a high net amount ofp-type dopant compared to the drain side of IGFET 140. The highsource-side p-type doping in channel zone 106 of IGFET 140 causes thethickness of the channel-side portion of the depletion region extendingalong source-body junction 110 to be reduced.

Source 102 and drain 104 are closer to each other in IGFET 140 than inIGFET 100. Accordingly, it is more likely that the depletion regionextending along source 102 will punch through to the depletion regionextending along drain 104 in IGFET 140 than in IGFET 100. However, thehigh amount of source-side p-type dopant in channel zone 106 of IGFET140 reduces the likelihood of punchthrough occurring in IGFET 140relative to an otherwise equivalent short re-channel IGFET lackingpocket portion 120.

The dopant concentrations along vertical lines 130, 132, and 136respectively through source 102, channel zone 106, and drain 104 inIGFET 140 are substantially the same as in IGFET 100. ConcentrationsN_(I), N_(T), and N_(N) shown in FIGS. 8-10 along vertical lines 130,132, and 136 thus apply to IGFET 140. Accordingly, IGFET 140 has ahypoabrupt vertical dopant profile in the portion of body material 108below drain 104. This causes the parasitic capacitance associated withdrain-body pn junction 112 to decrease, as further described below, sothat IGFET 140 has increased analog speed.

Source 102 can be longitudinally dopant graded to reduce its source(series) resistance R_(S). As discussed below, reducing sourceresistance R_(S) is particularly advantageous in analog IGFETapplications. This longitudinal dopant grading in source 102 typicallyinvolves configuring it as a main portion and a more lightly dopedlateral extension that terminates channel zone 106 along the uppersemiconductor surface. Drain 104 can be provided with a similarlongitudinal dopant grading to reduce to reduce hot-carrier injection.Providing both of S/D zones 102 and 104 with longitudinal dopant gradingis therefore advantageous regardless of whether zones 102 and 104respectively act as source and drain, the normal case, or respectivelyas drain and source.

FIG. 13 illustrates an asymmetric long n-channel IGFET 150 configured inaccordance with the invention to be especially suitable for high-speedanalog applications and, in particular, to have longitudinalsource/drain dopant grading for reducing source resistance R_(S) anddrain-side hot-carrier injection. IGFET 150 is arranged the same asIGFET 100 except that (a) n-type source 102 consists of a very heavilydoped main portion 102M and a more lightly doped lateral extension 102Eand (b) n-type drain 104 consists of a very heavily doped main portion104M and a more lightly doped lateral extension 104E. Although morelightly doped than n++ main S/D portions 102M and 104M, lateralextensions 102E and 104E are still heavily doped in sub-μm complementaryIGFET applications such as the present one. N+ lateral extensions 102Eand 104E terminate channel zone 106 along the upper semiconductorsurface. Gate electrode 128 extends partially over each of n+ lateralextensions 102E and 104E but typically not over n++ main source portion102M or n++ main drain portion 104M.

Main S/D portions 102M and 104M normally extend deeper below the uppersemiconductor surface respectively than lateral extensions 102E and104E. Consequently, source depth y_(S) and drain depth y_(D) in IGFET150 respectively are the depths of main source portion 102M and maindrain portion 104M. Pocket portion 120 extends under, and partiallyalongside, main source portion 102M so that drain depth y_(D) againnormally exceeds source depth y_(S). Also, pocket 120 extends under, andalongside, source extension 102E. As a result, drain extension 104Enormally extends deeper below the upper surface than source extension102E.

Maximum net dopant concentration N_(N) in n++ main source portion 102Malong the upper semiconductor surface is normally at least 1×10²⁰atoms/cm³, typically 4×10²⁰ atoms/cm³. Maximum net dopant concentrationN_(N) in n++ main drain portion 104M along the upper semiconductorsurface is normally at least 1×10²⁰ atoms/cm³, typically slightlygreater than 4×10²⁰ atoms/cm³ so as to slightly exceed the maximumupper-surface N_(N) concentration in main source portion 102M. Maximumnet dopant concentration N_(N) in n+ source extension 102E along theupper semiconductor surface is normally 1×10¹⁸-1×10¹⁹ atoms/cm³,typically 3×10¹⁸ atoms/cm³. Maximum net dopant concentration N_(N) in n+drain extension 104E along the upper semiconductor surface is typicallyslightly greater than 3×10¹⁸ atoms/cm³ so as to slightly exceed themaximum upper-surface N_(N) concentration in source extension 102E.

Subject to the longitudinal dopant grading in source 102 and drain 104,channel zone 106 is asymmetrically longitudinally dopant gradedsubstantially the same in IGFET 150 as in IGFET 100. FIGS. 14 a-14 c(collectively “FIG. 14”) present exemplary dopant concentrations alongthe upper semiconductor surface of IGFET 150 as a function oflongitudinal distance x for use in examining the longitudinal dopantgrading in source 102 and drain 104. FIG. 14 a depicts concentrationsN_(I), along the upper semiconductor surface, of the individualsemiconductor dopants that largely define regions 102M, 102E, 104M,104E, 120, and 124. Concentrations N_(T) of the total p-type and totaln-type dopants of regions 102M, 102E, 104M, 104E, 120, and 124 along theupper semiconductor surface are depicted in FIG. 14 b. FIG. 14 cillustrates net dopant concentration N_(N) along the upper semiconductorsurface.

FIG. 14 a is analogous to FIG. 7 a except that curves 102M′, 102E′,104M′, and 104E′ represent concentrations N_(I), along the uppersemiconductor surface, of the n-type dopant used to respectively formregions 102M, 102E, 104M, and 104E. FIG. 14 b is similarly analogous toFIG. 7 b subject to total concentrations N_(T) of the n-type dopantalong the upper semiconductor surface being represented by (a) curve102″ consisting of segments 102M″ and 102E″ respectively correspondingto main source portion 102M and source extension 102E and (b) curve 104″consisting of segments 104M″ and 104E″ respectively corresponding tomain drain portion 104M and drain extension 104E. FIG. 14 c is analogousto FIG. 7 c subject to (a) curve 102* that represents net dopantconcentration N_(N) in source 102 along the upper semiconductor surfacebeing formed with segments 102M* and 102E* respectively corresponding tomain source portion 102M and source extension 102E and (b) curve 104*that represents net dopant concentration N_(N) in drain 104 along theupper semiconductor surface being formed with segments 104M* and 104E*respectively corresponding to main drain portion 104M and drainextension 104E.

The longitudinal dopant grading in source 102 and drain 104 of IGFET 150reduces source resistance R_(S) and alleviates drain-side hot-carrierinjection but does not have any significant effect on the asymmetriclongitudinal dopant grading in channel zone 106. Accordingly, theasymmetric channel-zone dopant grading in IGFET 150 avoids punchthroughin largely the same way as in IGFET 100.

The configuration of well portion 116 and upper body-material remainder124 in IGFET 150 causes the vertical dopant profile through drain 104and into underlying body material 108 to be hypoabrupt substantially thesame as in IGFET 100. With vertical lines 130 and 136 respectively goingthrough n++ main source portion 102M and n++ main drain portion 104M,the vertical dopant concentration graphs of FIGS. 8-10 substantiallyapply to IGFET 150. The resulting reduced parasitic capacitanceassociated with drain-body pn junction 112 enables IGFET 150 to haveincreased analog speed. The reduction in source resistance R_(S) furtherenhances the analog performance in the manner discussed below.

Drain 104 can be vertically dopant graded to further reduce theparasitic capacitance associated with drain-body junction 112. Source102 can similarly be vertically dopant graded for reducing the parasiticcapacitance associated with source-body junction 110. The verticaldopant grading typically involves configuring each S/D zone 102 or 104as a main portion and a more lightly doped lower portion. The verticalsource/drain dopant grading can be combined with the above-mentionedlongitudinal dopant grading of source 102 and drain 104.

In the foregoing regard, FIG. 15 illustrates an asymmetric longn-channel IGFET 160 configured in accordance with the invention to beespecially suitable for high-speed analog applications. IGFET 160 isprovided both with source/drain longitudinal dopant grading for reducingsource resistance R_(S) and drain-side hot-carrier injection and withsource/drain vertical dopant grading for reducing source/drain parasiticcapacitances. IGFET 160 is arranged the same as IGFET 150 except that(a) source 102 further includes a lower portion 102L more lightly dopedthan main source portion 102M and (b) drain 104 further includes a lowerportion 104L more lightly doped than main drain portion 104M. Lowersource portion 102L and lower drain portion 104L are heavily dopedn-type.

Source depth y_(S) and drain depth y_(D) in IGFET 160 are respectivelythe depths of n+ lower source portion 102L and n+ lower drain portion104L, since they respectively underlie n++ main source portion 102M andn++ main drain portion 104M. Pocket portion 120 extends below lowersource portion 102L. Consequently, drain depth y_(D) once again normallyexceeds source depth y_(S).

Source 102 and drain 104 of IGFET 160 respectively include n+ lateralsource extension 102E and n+ lateral drain extension 104E for achievinglongitudinal source-drain dopant grading. See FIG. 15. The longitudinalsource/drain dopant grading in IGFET 160 has substantially the samecharacteristics as in IGFET 150. Accordingly, the longitudinalupper-surface dopant concentration graphs of FIG. 14 and the associateddescription of FIG. 14 in connection with IGFET 150 apply to IGFET 160.Since the longitudinal source/drain dopant grading in IGFET 150, andthus in IGFET 160, does not have any significant effect on theasymmetric longitudinal dopant grading in channel zone 106, theasymmetric channel-zone dopant grading in IGFET 160 avoids punchthroughin IGFET 160 in substantially the same way as in IGFET 100.

An understanding of the vertical dopant grading in IGFET 160 isfacilitated with the assistance of FIGS. 16 a-16 c (collectively “FIG.16”) and FIGS. 17 a-17 c (collectively “FIG. 17”) which presentexemplary dopant concentrations as a function of depth y along verticallines 130 and 136 respectively through source 102 and drain 104including respectively through lower source portion 102L and lower drainportion 104L. FIGS. 16 a and 17 a specifically illustrate concentrationsN_(I), respectively along lines 130 and 136, of the individualsemiconductor dopants that vertically define regions 102M, 102L, 104M,104L, 114, 116, 120, and 124. Concentrations N_(T) of the total p-typeand total n-type dopants in regions 102M, 102L, 104M, 104L, 114, 116,120, and 124 along lines 130 and 136 are respectively depicted in FIGS.16 b and 17 b. FIGS. 16 c and 17 c respectively illustrate net dopantconcentration N_(N) along lines 130 and 136.

FIGS. 16 a and 17 a are respectively analogous to FIGS. 8 a and 10 aexcept that (a) curves 102M′ and 102L′ represent concentrations N_(I)along vertical line 130 of the n-type dopant used to respectively formmain source portion 102M and lower source portion 104L and (b) curves104M′ and 104L′ represent concentrations N_(I) along vertical line 136of the n-type dopant used to respectively form main drain portion 104Mand lower drain portion 104L. Similarly, FIGS. 16 b and 17 b arerespectively analogous to FIGS. 8 b and 10 b subject to (a)concentration N_(T) of the total n-type dopant along line 130 beingrepresented by curve 102″ consisting here of segments 102M″ and 102L″respectively corresponding to main source portion 102M and lower sourceportion 102L and (b) concentration N_(T) of the total n-type dopantalong line 136 being represented by curve 104″ consisting here ofsegments 104M″ and 104L″ respectively corresponding to main drainportion 104M and lower drain portion 102L.

FIGS. 16 c and 17 c are respectively analogous to FIGS. 8 c and 10 csubject to (a) curve 102* that represents net dopant concentration N_(N)in source 102 along line 130 being formed here with segments 102M* and102L* respectively corresponding to main source portion 102M and lowersource portion 102L and (b) curve 104* that represents net dopantconcentration N_(N) in drain 104 along line 136 being formed here withsegments 104M* and 104L* respectively corresponding to main drainportion 104M and lower drain portion 102L. Additionally, the dopantconcentrations along vertical lines 132 and 134 through channel zone 106of IGFET 160 are substantially the same as in IGFET 100. Hence,concentrations N_(I), N_(T), and N_(N) shown in FIG. 9 along verticallines 132 and 134 apply to IGFET 160.

Subject to the preceding comments about the vertical dopant grading insource 102 and drain 104 of IGFET 160, the configuration of well portion116 and pocket portion 120 in IGFET 160 is substantially the same as inIGFET 100. Accordingly, the vertical dopant profile below drain 104 issubstantially the same in IGFET 160 as in IGFET 100. For this reason,the parasitic capacitance associated with drain-body junction 112 isreduced in IGFET 160, thereby enabling it to have increased analogspeed. The vertical dopant gradings in source 102 and drain 104 enableIGFET 160 to have even greater analog speed by reducing (or furtherreducing) the parasitic capacitance along source-body junction 110 andby further reducing the parasitic capacitance along drain-body junction112. The longitudinal dopant gradings in source 102 and drain 104 ofIGFET 160 reduce source resistance R_(S) while simultaneouslyalleviating drain-side hot-carrier injection.

FIGS. 18 a-18 c illustrate versions 170, 180, and 190 of respectiveasymmetric long re-channel IGFETs 100, 150, and 160 in which pocketportion 120 extends to a lesser depth below the upper semiconductorsurface than source 102 and drain 104. For long n-channel IGFET 180 or190 whose source 102 and drain 104 respectively include source extension102E and drain extension 104E, pocket portion 120 extends belowextensions 102E and 104E.

The p type section of the boundary of pocket portion 120 in each IGFETs170, 180, or 190 is, as explained above in connection with IGFET 100,defined as the location where the concentration of the p-type pocketdopant equals the concentration of the p-type background dopant. Thetotal p-type dopant concentration along the p-type section of theboundary of pocket portion 120 is then twice the background dopantconcentration in IGFET 170, 180, or 190. Hence, some of the p-typepocket dopant is present in source 102 of IGFET 170, 180, or 190 at adepth below that illustrated for pocket portion 120 in FIGS. 18 a-18 c.This additional p-type pocket dopant in source 102 cancels (compensatesfor) some of the n-type dopant that defines source 102 along its lowersurface. Accordingly, drain depth y_(D) in IGFET 170, 180, or 190exceeds source depth y_(S), albeit by a lesser amount than in IGFET 100.

Channel zone 106 of each of IGFETs 170, 180, and 190 is asymmetricallylongitudinally dopant graded substantially as described aboverespectively for IGFETs 100, 150, and 160. In this regard, dopantconcentrations N_(I), N_(T), and N_(N) along the upper semiconductorsurface for IGFET 170 are also substantially respectively represented inFIG. 7. FIG. 14 substantially presents dopant concentrations N_(I),N_(T), and N_(N) along the upper surface for IGFETs 180 and 190.Punchthrough is thus avoided in IGFETs 170, 180, and 190 in the mannerdescribed above for IGFET 100.

Each of IGFETs 170, 180, and 190 has a hypoabrupt vertical dopantprofile below drain 104 substantially as described above for IGFETs 100,150, and 160. FIG. 10 respectively also substantially presentsconcentrations N_(I), N_(T), and N_(N) along vertical line 136 throughdrain 104 for each of IGFETs 170 and 180. Concentrations N_(I), N_(T),and N_(N) along vertical line 136 through drain 104 for IGFET 190 aresubstantially represented in FIG. 17. The parasitic capacitance alongdrain-body junction 112 in each IGFET 170, 180, or 190 is therebyreduced, as described above for IGFET 100, to enable each IGFET 170,180, or 190 to have increased analog speed.

FIGS. 19 a-19 c (collectively “FIG. 19”) and FIGS. 20 a-20 c(collectively “FIG. 20”) present exemplary dopant concentrations as afunction of depth y along vertical line 130 through source 102 forIGFETs 170, 180, and 190. FIG. 19 applies to IGFETs 170 and 180. FIG. 20applies to IGFET 190. FIGS. 19 a and 20 a specifically illustrateconcentrations N_(I) of the individual semiconductor dopants thatvertically define regions 102, 114, 116, and 120 along line 130.Concentrations N_(T) of the total p-type and total n-type dopants inregions 102, 114, 116, and 120 along line 130 are depicted in FIGS. 19 band 20 b. FIGS. 19 c and 20 c respectively illustrate net dopantconcentration N_(N) along line 130.

As indicated by the variation of curve segments 116″ and 124″ in FIGS.19 b and 20 b, concentration N_(T) of the total p-type dopant in theportion of body material 108 below source 102 decreases hypoabruptly byat least a factor of 10 in moving upward along vertical line 130 tosource 102 from the subsurface location of the maximum concentration ofthe p-type dopant in well portion 116. Configuring pocket portion 120 tobe shallower than source 102 and drain 104 thus results in a hypoabruptvertical profile for the total p-type dopant in the portion of bodymaterial 108 below source 102. This arises because much less of thep-type dopant is situated below source 102 in IGFETs 170, 180, and 190than in IGFETs 100, 150, and 160. Curve segment 120″ which representsconcentration N_(T) of the total p-type pocket dopant along line 130through source 102 in FIGS. 8 b, 16 b, 19 b, and 20 b is largely locatedabove source depth y_(S) in FIGS. 19 b and 20 b but extends considerablybelow depth y_(S) in FIGS. 8 b and 16 b.

The hypoabrupt vertical dopant profile in the portion of body material108 below source 102 of IGFETs 170, 180, or 190 is quite similar to thehypoabrupt vertical dopant profile in the portion of body material 108below drain 104 of IGFET 170, 180, or 190 and thus below drain 104 ofIGFET 100, 150, or 160. Compare combined curve segment 116″/120″ alongvertical line 130 through source 102 in FIGS. 19 b and 20 b withcombined curve segment 116″/120″ along vertical line 136 through drain104 in FIGS. 10 b and 17 b. Similar to what occurs in the portion ofbody material 108 below drain 104, concentration N_(T) of the totalp-type dopant in the portion of body material 108 below source 102 ineach IGFET 170, 180, or 190 preferably decreases by at least a factor of20, more preferably by at least a factor of 40, even more preferably byat least a factor of 80, typically by a factor in the vicinity of 100,in moving from the subsurface location of the maximum p-type wellconcentration up to the very heavily doped material of source 102. InIGFET 170, 180, or 190, the hypoabrupt vertical dopant profile in theportion of body material 108 below the very heavily doped material ofsource 102 causes the parasitic capacitance associated with source-bodyjunction 110 to decrease. The analog speed of IGFET 170, 180, or 190 isfurther increased.

Dopant concentrations N_(I), N_(T), and N_(N) along vertical line 134through channel zone 106 of IGFET 170, 180, or 190 appear substantiallyas shown in FIG. 9. Dopant concentrations N_(I), N_(T), and N_(N) alongvertical line 132 through channel zone 106, including pocket portion120, of IGFET 170, 180, or 190 are similar to what is illustrated inFIG. 9 except that curve 120″ for concentration N_(I) of the p-typepocket dopant along line 132 for IGFET 170, 180, or 190 is similar tocurve 120′ along line 130 in FIG. 19 a or 20 a.

For the purpose of simplicity in describing IGFETs 100, 140, 150, 160,170, 180, and 190, it was assumed above that the concentration of thep-type background dopant is substantially constant throughout thesemiconductor material containing any of IGFETs 100, 140, 150, 160, 170,180, and 190. However, the concentration of the p-type background dopantcan vary as long as the peak value of the p-type background dopant isrelatively low compared to the concentration of the other p-type dopant.

Well portion 116 of body material 108 in each of IGFETs 100, 140, 150,160, 170, 180, and 190 is of the same conductivity type as the directlyunderlying lightly doped semiconductor material (lower body-materialportion 114). As indicated below in connection with the fabricationprocess of FIGS. 31 a-31 o and 31 p.1-31 r.2, this situation normallyarises when p+ well portion 116 and p+ pocket portion 120 are created ina starting region of lightly doped p-type semiconductor material. Thedopant concentration in the bulk of upper body-material remainder 124thereby largely equals the low background dopant concentration of the p−starting region.

Alternatively, the semiconductor material directly underlying wellportion 116 can be of opposite conductivity type to well 116. Since wellportion 116 is p type, the semiconductor material directly underlyingwell 116 is then n type. This alternative typically arises when p+ well116 and p+ pocket 120 are created in a starting region of n-typesemiconductor material, normally lightly doped at a relatively uniformnet background concentration. In one embodiment, the portion of thestarting n-type region intended to become upper body-material portion118, i.e., the portion of the starting n-type region situated above wellportion 116 (or above the intended location for well 116), is doped withp-type compensating dopant to an absolute concentration greater than then-type background dopant concentration of the starting n-typesemiconductor region so as to cause all of upper body-material portion118 to be p type. In another embodiment, the portion of the startingn-type region intended to become upper body-material portion 118 isconverted to p-type conductivity by updiffusion of part of the p-typewell dopant in well portion 116.

The minimum value of net concentration N_(N) of the p-type compensatingor well dopant in upper body-material portion 118 can be in the vicinityof n-type background dopant concentration. However, in order to ensurethat all of body-material portion 118 is p type, the minimum value ofconcentration N_(N) of the p-type compensating or well dopant in portion118 is normally a significant amount greater than, e.g., at least twiceas great as, the n-type background dopant concentration. The minimumvalue of concentration N_(N) of the p-type compensating or well dopantin the bulk of body-material portion 118 outside pocket portion 120 isthus normally significantly greater than the n-type background dopantconcentration.

FIG. 21 illustrates, in accordance with the invention and analogous toFIG. 6, a variation 100V of asymmetric long n-channel IGFET 100 in whichp− lower body-material portion 114 is replaced with a lightly dopedn-type lower region 192 that forms a lower pn junction 194 with p+ wellportion 116. Since lower region 192 is not of p-type conductivity,p-type body material 108 in IGFET 100V consists of well portion 116 andan upper portion 196 that replaces upper body-material portion 118 ofIGFET 100. Part of upper body-material portion 196 of IGFET 100V isformed by p+ pocket portion 120. The remainder of upper body-materialportion 196, i.e., the part outside pocket portion 120, is indicated asitem 198 in FIG. 21. Upper body-material remainder 198 is lightly dopedp type at a somewhat higher net concentration than n− lower portion 192.The light p-type doping of body-material remainder 198 is achieved withthe above-mentioned p-type compensating dopant. Aside from the precedingdifferences and the resultant dopant concentration differences, IGFET100V is configured and constituted substantially the same as IGFET 100.

IGFET 100V has the following features similar to those of IGFET 100: (a)an asymmetric longitudinal dopant grading in channel 106 and (b) ahypoabrupt vertical dopant profile in the portion of body material 108underlying drain 104. An understanding of these features of IGFET 100V,including how they can respectively differ somewhat from those of IGFET100, is facilitated with the assistance of FIGS. 22 a-22 c (collectively“FIG. 22”), FIGS. 23 a-23 c (collectively “FIG. 23”), and FIGS. 24 a-24c (collectively “FIG. 24”). FIG. 22 presents exemplary dopantconcentrations along the upper semiconductor surface of IGFET 100V as afunction of longitudinal distance x. Exemplary dopant concentrationsalong vertical line 130 through source 102 of IGFET 100V are presentedin FIG. 23. FIG. 24 presents exemplary dopant concentrations alongvertical line 136 through drain 104 of IGFET 100V.

FIGS. 22 a, 23 a, and 24 a illustrate concentrations N_(I) of theindividual semiconductor dopants that define regions 102, 104, 116, 120,192, 196, and 198. Curves 192′ and 198′ specifically respectivelyrepresent concentrations N_(I) of the n-type background dopant and thep-type compensating dopant that respectively define n− lower region 192and p− upper body-material remainder 198. Item 194 ^(#) indicates wherenet dopant concentration N_(N) goes to zero below well portion 116 andthus indicates the location of lower pn junction 194.

Concentrations N_(T) of the total p-type and total n-type dopants inregions 102, 104, 116, 120, 192, 196, and 198 are depicted in FIGS. 22b, 23 b, and 24 b. Curve segments 192″ and 198″ in FIGS. 22 b, 23 b, and24 b respectively correspond to n− lower region 192 and p− upperbody-material remainder 198. FIGS. 22 c, 23 c, and 24 c illustrateconcentrations N_(N) of the net p-type dopant and net n-type dopantvariously in regions 102, 104, 116, 120, 192, 196, and 198. Curvesegments 192* and 198* in FIGS. 22 c, 23 c, and 24 c respectivelycorrespond to n− lower region 192 and p− upper body-material remainder198.

FIGS. 22-24 represent an example in which (a) concentration N_(I) of then-type background dopant in IGFET 100V approximately equalsconcentration N_(I) of the p-type background dopant in IGFET 100, (b)concentration N_(I) of the p-type compensating dopant along the uppersemiconductor surface of IGFET 100V is 2-3 times concentration N_(I) ofthe n-type background dopant in IGFET 100V, and (c) the maximum value ofconcentration N_(I) of the p-type compensating dopant is 2-3 timesconcentration N_(I) of the p-type compensating dopant along the uppersemiconductor surface and thus is 4-9 times concentration N_(I) of then-type background dopant. Aside from these differences, concentrationsN_(I) of the other dopants in IGFET 100V are respectively largely thesame as in IGFET 100.

More particularly, concentration N_(I) of the p-type pocket dopant inIGFET 100V varies longitudinally in substantially the same way as inIGFET 100. The variation of curve segment 120′ in FIG. 22 a shows that,in moving longitudinally from source 102 along the upper semiconductorsurface of IGFET 100V into channel zone 106, concentration N_(I) of thep-type pocket dopant is at an approximately constant upper-surface levelpartway into zone 106 and then drops from that level essentially to zeroat a location between source 102 and drain 104.

The total p-type dopant in channel zone 106 along the uppersemiconductor surface of IGFET 100V is the sum of the pocket andcompensating dopants. This differs from IGFET 100 where the total p-typedopant in channel zone 106 along the upper semiconductor surface is thesum of the pocket and background dopants. Since concentration N_(I) ofthe p-type compensating dopant in the illustrated example is 2-3 timesconcentration N_(I) of the n-type background dopant and thus is 2-3times concentration N_(I) of the p-type background dopant in IGFET 100,the minimum value of concentration N_(T) of the total p-type dopantalong the upper surface of IGFET 100V in the illustrated example is 2-3times the minimum value of concentration N_(T) of the total p-typedopant along the upper surface of IGFET 100.

Item 106″ in FIG. 22 b represents the channel-zone portions of curvesegments 120″ and 198″. Similar to what occurs in IGFET 100, thevariation in curve 106″ here shows that concentration N_(T) of the totalp-type dopant in channel zone 106 along the upper semiconductor surfaceof IGFET 100V is lower where zone 106 meets drain 104 than where zone106 meets source 102. Concentration N_(T) of the total p-type dopant inchannel zone 106 of IGFET 100V is normally at least a factor of 10lower, preferably at least a factor of 20 lower, more preferably atleast a factor of 50 lower, typically a factor in the vicinity of 100lower, at drain-body junction 112 along the upper surface than atsource-body junction 110 along the upper surface. The reason why thetypical value for this concentration difference is a factor in thevicinity of 100 in IGFET 100V, rather than a factor of more than 100 ascan arise in IGFET 100, is that the minimum value of concentration N_(T)of the total p-type dopant along the upper surface is 2-3 times higherin the illustrated example of IGFET 100V than in IGFET 100.

Referring to FIG. 22 c, item 106* represents the combination ofchannel-zone curve segments 120* and 198* here. Subject to segment 198*of curve 106* for IGFET 100V being slightly higher than segment 124* ofcurve 106* for IGFET 100 in FIG. 7 a, curves 106* in FIGS. 7 a and 22 aare quite similar. Accordingly, the source side of channel zone 106 inIGFET 100V has a high net amount of p-type dopant compared to the drainside. The thickness of the channel-side portion of the depletion regionalong source-body junction 110 is thereby reduced in IGFET 100V. Inaddition, the high p-type dopant concentration along the source side ofchannel zone 106 in IGFET 100V shields source 102 from the comparativelyhigh electric field in drain 104 for the (field-line termination)reasons described above in connection with IGFET 100. Punchthrough isavoided in IGFET 100V.

The p-type dopant in the portions of body material 108 below source 102and drain 104 of IGFET 100V consists of the well and compensatingdopants as indicated respectively by curves 116′ and 198′ in FIGS. 23 aand 24 a and, for source 102, the pocket dopant as indicated by curve120′ in FIG. 23. The variation of curve 198′ in FIGS. 23 a and 24 ashows that concentration N_(I) of the p-type compensating dopant reachesa maximum value close to the bottoms of source 102 and drain 104. Thismaximum value is 4-9 times concentration N_(I) of the n-type backgrounddopant in the particular example of FIGS. 23 a and 24 a. ConcentrationN_(I) of the p-type compensating dopant in the example of FIGS. 23 a and24 a drops essentially to zero at a depth less than depth y_(W) of themaximum p-type dopant concentration in well portion 116.

The total p-type dopant in the portion of body material 108 below drain104 of IGFET 100V is indicated by curve segment 116″ and its (upward)extension 198″ in FIG. 24 b. Because concentration N_(I) of the p-typecompensating dopant drops essentially to zero at a depth less than depthy_(W), concentration N_(T) of the total p-type dopant in thebody-material portion below drain 104 reaches a maximum value at asubsurface location substantially equal to y_(W). As occurs in IGFET100, the variation in combined curve segment 116″/198″ in FIG. 24 bshows that concentration N_(T) of the total p-type dopant in the portionof body material 108 below drain 104 of IGFET 100V decreaseshypoabruptly by at least a factor of 10 in moving upward to drain 104from the subsurface location of the maximum concentration of the totalp-type dopant in well portion 116.

Concentration N_(T) of the total p-type dopant in the portion of bodymaterial 108 below drain 104 of IGFET 100V in the particular example ofFIG. 24 b typically decreases by a factor in the vicinity of 15 inmoving from the location of the maximum p-type well concentration up todrain 104. The reason why the typical value for this hypoabruptconcentration decrease is in the vicinity of a factor of 15 in IGFET100V rather than in the vicinity of a factor 100 as arises in IGFET 100is that concentration N_(I) of the p-type compensating dopant at thebottom of drain 104 in IGFET 100V in the example of FIG. 24 a is 4-9times concentration N_(I) of the p-type background dopant at the bottomof drain 104 in IGFET 100. However, the vertical dopant profile for thep-type compensating dopant can be lowered to reduce the value ofconcentration N_(I) of the p-type compensating dopant at the bottom ofdrain 104 while still ensuring that all of upper body-material portion196 is p type. Concentration N_(T) of the total p-type dopant in thebody material portion below drain 104 in IGFET 100V can readily decreaseby at least a factor of 20, and typically by at least a factor of 40, inmoving upward to drain 104 from the location of the maximumconcentration of the total p-type dopant in well portion 116.

As represented by the combination of curve segments 116* and 198*, FIG.24 c shows that concentration N_(N) of the net p-type dopant in theportion of body material 108 below drain 104 in IGFET 100V variesvertically in a similar manner to concentration N_(T) of the totalp-type dopant in the body-material portion below drain 104 except thatconcentration N_(N) in the body-material portion below drain 104 dropsto zero at pn junctions 112 and 194. Due to the hypoabrupt verticaldopant profile in the body-material portion below drain 104 of IGFET100V, the parasitic capacitance associated with drain-body junction 112is again decreased for the reasons discussed further below. Although thedecrease in the parasitic capacitance along junction 104 may be less inIGFET 100V than in IGFET 100, IGFET 100V still has increased analogspeed.

The presence of the p-type compensating dopant at a concentrationgreater than that of the p-type background dopant in IGFET 100 hasconsiderably lesser effect on the vertical dopant profile through source102 of IGFET 100V than along the vertical dopant profile through drain104 of IGFET 100V because the p-type pocket dopant is also present belowsource 102 in IGFET 100V. As is evident from comparing FIG. 23 to FIG.24, the comments made above about the vertical dopant profile throughsource 102 of IGFET 100 apply generally to the vertical dopant profilethrough source 102 of IGFET 100V.

FIG. 25 illustrates, in accordance with the invention and analogous toFIG. 13, a variation 150V of asymmetric long n-channel IGFET 150 inwhich n− lower portion 192 again replaces p-lower body-material portion114. IGFET 150V likewise contains p-type upper body-material portion 196that replaces upper body-material portion 118 of IGFET 150. P− remainder198 of upper body-material portion 196 is again at a slightly higher netdopant concentration than n− lower portion 192. As with IGFET 100V, thelight p-type doping of upper body-material remainder 198 in IGFET 150Vis achieved with the p-type compensating dopant. Subject to the presenceof n− lower portion 192 and p− upper body-material remainder 198, IGFET150V is configured substantially the same as IGFET 150 so as to havelongitudinal source/drain dopant grading for reducing source resistanceR_(S) and drain-side hot-carrier injection.

FIGS. 26 a-26 c (collectively “FIG. 26”) present exemplary dopantconcentrations along the upper semiconductor surface of IGFET 150V foruse in examining the longitudinal dopant grading in source 102 and drain104. Concentrations N_(I), along the upper surface, of the individualsemiconductor dopants that largely define regions 102M, 102E, 104M,104E, 120, 192, and 198 are depicted in FIG. 26 a. FIG. 26 b illustratesconcentrations N_(T) of the total p-type and n-type dopants of regions102M, 102E, 104M, 104E, 120, 192, and 198 along the upper surface. Netconcentration N_(N) along the upper surface is illustrated in FIG. 26 c.

FIG. 26 repeats FIG. 14 subject to respective modifications of FIG. 22to account for n-lower portion 192 and p− upper body-material remainder198. The longitudinal dopant grading in source 102 and drain 104 ofIGFET 150V does not have any significant effect on the asymmetriclongitudinal dopant grading in channel zone 106. The asymmetricchannel-zone dopant grading in IGFET 150V avoids punchthrough in largelythe same way as in IGFET 150 and thus in largely the same way as inIGFET 100.

The configuration of p+ well portion 116 and p− upper body-materialremainder 198 in IGFET 150V causes the vertical dopant profile throughdrain 104 and into underlying body material 108 to be hypoabruptsubstantially the same as in IGFET 150. The vertical dopantconcentration graphs of FIGS. 23 and 24 along vertical lines 130 and 136substantially apply to IGFET 150V. Drain-body junction 112 in IGFET 150Vhas reduced parasitic capacitance which, although typically not reducedas much as in IGFET 150, enables IGFET 150V to have increased analogswitching speed.

FIGS. 27 a and 27 b illustrate, in accordance with the invention andrespectively analogous to FIGS. 18 a and 18 b, variations 170V and 180Vof respective asymmetric long n-channel IGFETs 170 and 180 in which n−lower portion 192 replaces p− lower body-material portion 114. EachIGFET 170V or 180V likewise contains p-type upper body-material portion196 that replaces upper body-material portion 118 of IGFET 170 or 180.P− remainder 198 of upper body-material portion 196 is once again at aslightly higher net dopant concentration than n− lower portion 192. Aswith IGFETs 100V and 150V, the light p-type doping of upperbody-material remainder 198 in each IGFET 170V or 180V is achieved withthe p-type compensating dopant. Subject to the presence of n− lowerportion 192 and p− upper body-material remainder 198, pocket portion 120of each IGFET 170V or 180V extends to a lesser depth below the uppersemiconductor surface than source 102 or drain 104. IGFET 180V also hasthe longitudinal source/drain dopant grading of IGFET 150V for reducingsource resistance R_(S) and drain-side hot-carrier injection.

Channel zone 106 of each of IGFETs 170V and 180V is asymmetricallylongitudinally dopant graded substantially as described aboverespectively for IGFETs 100V and 150V. FIG. 22 substantially presentsconcentrations N_(I), N_(T), and N_(N) along the upper semiconductorsurface for IGFET 170V. Punchthrough is thus avoided in IGFET 170V forthe reasons described above in connection with IGFET 100V.Concentrations N_(I), N_(T), and N_(N) along the upper semiconductorsurface of IGFET 180V are substantially respectively represented in FIG.26. IGFET 180V avoids punchthrough as described above for IGFET 170V andthus as described above for IGFET 100V.

Each of IGFETs 170V and 180V has a hypoabrupt vertical dopant profilethrough drain 104 as described above for IGFET 100V. FIG. 24 alsopresents concentrations N_(I), N_(T), and N_(N) along vertical line 136through drain 104 for each IGFET 170V or 180V. As a result, theparasitic capacitance along drain-body junction 112 in each IGFET 170Vor 180V is reduced for the reasons described above in connection withIGFET 100V. IGFETs 170V and 180V thereby have increased analog speed.

FIGS. 28 a-28 c (collectively “FIG. 28”) respectively presentconcentrations N_(I), N_(T), and N_(N) along vertical line 130 throughsource 102 for each IGFET 170V or 180V. As indicated by the variation ofcurve segments 116″ and 198″ in FIG. 28 b, concentration N_(T) of thetotal p-type dopant in the portion of body material 108 below source 102decreases hypoabruptly by at least a factor of 10 in moving upward alongline 130 to source 102 from the subsurface location of the maximumconcentration of the p-type dopant in well portion 116. As occurs inIGFETs 170 and 180, configuring pocket portion 120 to be shallower thansource 102 and drain 104 in IGFETs 170V and 180V results in a hypoabruptvertical profile for the p-type dopant in the body-material portionbelow source 102.

The hypoabrupt vertical dopant profile in the portion of body material108 below source 102 for each IGFET 170V or 180V is quite similar to thehypoabrupt vertical dopant profile in the portion of body material 108below drain 104. In the particular example of FIG. 28, concentrationN_(T) of the total p-type dopant in the body-material portion belowsource 102 of IGFET 170V or 180V decreases by a typical factor in thevicinity of 15 in moving from the location of the maximum p-type wellconcentration up to source 102. Although this typical factor of 15 isconsiderably less than the corresponding typical factor of 100 whichoccurs in IGFET 170 or 180, the vertical dopant profile for the p-typecompensating dopant can be lowered. Similar to what was said above aboutthe vertical dopant profile through drain 104 of IGFET 100V,concentration N_(T) of the total p-type dopant in the body-materialportion below source 102 of IGFET 170V or 180V can readily decrease byat least a factor of 20, typically by at least a factor of 40, in movingupward to source 102 from the location of the maximum concentration ofthe total p-type dopant in well portion 116.

The hypoabrupt vertical dopant profile in the portion of body material108 below source 102 of IGFET 170V or 180V causes the parasiticcapacitance associated with source-body junction 110 to decrease, albeitby typically a smaller amount than in IGFET 170 or 180. Consequently,the analog speed of each IGFET 170V or 180V is further increased.

Variations of IGFETs 140, 160, and 190 can be provided with n− lowerregion 192 and p-upper body-material remainder 198 (or p-type upperbody-material portion 196) in the same way that regions 192 and 198 (or196) are provided in IGFETs 100V, 150V, 170V, and 180V. These asymmetriclong n-channel variations of IGFETs 140, 160, and 190 are respectivelyreferred to below as IGFETS 140V, 160V and 190V.

Complementary IGFET Structures Suitable for Mixed-Signal Applications

Short-channel versions of long-channel IGFETs 150, 160, 170, 180, 190,100V, 150V, 160V, 170V, 180V, and 190V can be produced in accordancewith the invention by appropriately reducing the channel length.P-channel IGFETs can likewise be produced in accordance with theinvention by reversing the conductivity types of the semiconductorregions of IGFETs 100, 140, 150, 160, 170, 180, 190, 100V, 140V, 150V,160V, 170V, 180V, and 190V, including the short-channel variations ofIGFETs 150, 160, 170, 180, 190, 150V, 160V, 170V, 180V, and 190V.

N-channel IGFETs 100, 140, 150, 160, 170, 180, 190, 100V, 140V, 150V,160V, 170V, 180V, and 190V, including the short-channel variations ofIGFETs 150, 160, 170, 180, 190, 150V, 160V, 170V, 180V, and 190V, andthe p-channel IGFETs can be variously provided in the same semiconductorstructures to produce a complementary-IGFET semiconductor architectureespecially suitable for high-speed analog applications. For instance,one or more of n-channel IGFETs 100, 140, 150, 160, 170, 180, and 190can be combined with one or more p-channel variations of IGFETs 100V,140V, 150V, 160V 170V, 180V, and 190V. The complementary-IGFET structureis then created from lightly doped p-type semiconductor material usingp− lower body-material portion 114 as the p-type equivalent of n− lowerportion 192 for each p-channel variation of IGFET 100V, 140V, 150V, 160V170V, 180V, or 190V. Alternatively, one or more of n-channel IGFETs100V, 140V, 150V, 160V 170V, 180V, and 190V can be combined with one ormore p-channel variations of IGFETs 100, 140, 150, 160, 170, 180, and190 fabricated from lightly doped n-type semiconductor material using n−lower portion 192 as the n-type equivalent of p− lower body-materialportion 114 for each p-channel variation of IGFET 100, 140, 150, 160,170, 180, or 190.

IGFETs, both n-channel and p-channel, particularly suitable for digitalcircuitry can also be provided in the semiconductor structure. Bipolartransistors, npn and/or pnp, can be variously provided in thesemiconductor structure. The resulting semiconductor architecture isthus suitable for mixed signal applications.

FIGS. 29.1 and 29.2 (collectively “FIG. 29”) depict two portions of acomplementary-IGFET semiconductor structure configured according to theinvention so as to be especially suitable for mixed-signal applications.The complementary-IGFET structure of FIG. 29 is created from a dopedmonosilicon semiconductor body having lower p− body-material portion114. A patterned field region 200 of electrically insulating material,typically consisting primarily of silicon oxide, is recessed into theupper surface of the semiconductor body to define a group of laterallyseparated active semiconductor islands. Four such islands 202, 204, 206,and 208 appear in FIG. 29.

Four long-channel IGFETs 210, 220, 230, and 240 are formed along theupper semiconductor surface respectively at the locations of islands202, 204, 206, and 208. IGFETs 210 and 220 in FIG. 29.1 are asymmetricdevices primarily intended for high-speed analog applications. IGFETs230 and 240 in FIG. 29.2 are symmetric devices primarily intended fordigital applications. IGFETs 210 and 230 are n-channel devices. IGFETs220 and 240 are p-channel devices.

Asymmetric n-channel IGFET 210 is an implementation of long n-channelIGFET 180 of FIG. 18 b and contains all the regions of IGFET 180.Accordingly, IGFET 210 has a hypoabrupt vertical dopant profile belowdrain 104 substantially as described above for IGFET 180 and thus asdescribed above for IGFET 100. Similarly, channel zone 106 of IGFET 210is asymmetrically longitudinally dopant graded as described above forIGFET 180 and therefore substantially as described above for IGFET 150.

Source 102, drain 104, and channel zone 106 of n-channel IGFET 210 aresituated in island 202. In addition to the regions depicted in FIG. 18b, IGFET 210 contains a pair of electrically insulating sidewall spacers250 and 252 situated along the opposite transverse sidewalls of gateelectrode 128. Metal silicide layers 254, 256, and 258 are respectivelysituated along the tops of source 102, drain 104, and gate electrode128.

Asymmetric p-channel IGFET 220 is an implementation of a p-channelversion of long re-channel IGFET 180V in which n− lower portion 192 isreplaced with p− lower body-material portion 114. IGFET 220 has a p-typesource 262 and a p-type drain 264 separated by an n-type channel zone266 of n-type body material 268 consisting of a heavily doped wellportion 276 and an upper portion 278. As an implementation of ap-channel version of n-channel IGFET 180V, p-channel IGFET 220 has ahypoabrupt vertical dopant profile below drain 264 substantially thesame, subject to reversal of the conductivity types, as that describedabove for n-channel IGFET 180V and thus as that described above forn-channel IGFET 100V. Similarly, channel zone 106 of p-channel IGFET 210is asymmetrically longitudinally dopant graded substantially the same,again subject to reversal of the conductivity types, as that describedabove for n-channel IGFET 180V and therefore as that described above forn-channel IGFET 150V.

Source 262, drain 264, and channel zone 266 of p-channel IGFET 220 aresituated in island 204. Each p-type S/D zone 262 or 264 consists of avery heavily doped main portion 262M or 264M and a more lightly doped,but still heavily doped, lateral extension 262E or 264E for reducingsource resistance R_(S) and drain-side hot-carrier injection. P+ lateralextensions 262E and 264E terminate channel zone 266 along the uppersemiconductor surface.

A heavily doped pocket portion 280 of n-type upper body-material portion278 extends along source 262, primarily source extension 262E. As withpocket portion 120 in IGFET 210, n+ pocket portion 280 extends deeperbelow the upper semiconductor surface than p+ source extension 262E butnot as deep as p++ main source portion 262M. The remainder 284 of n-typeupper body-material portion 278 is lightly doped and extends along drain264. N+ well portion 276, n+ pocket portion 280, and n− upperbody-material remainder 284 in IGFET 220 typically have largely the samelongitudinal and vertical doping characteristics respectively as p+ wellportion 116, p+ pocket portion 120, and p− upper body-material remainder198 in IGFET 180V with the conductivity types reversed. IGFET 220thereby avoids punchthrough and has reduced parasitic capacitances alongthe source-body and drain-body pn junctions.

In a variation of IGFET 220 described below in connection with FIGS. 32a-32 c and 33 a-33 f, n− upper body-material remainder 284 isessentially simply an extension of n+ well portion 276. The light n-typedoping of n− upper body-material remainder 284 in this variation isproduced by updiffusion of part of the n-type dopant used to form n+well 276 in order to avoid a separate dopant-introduction step forcausing remainder 284 to be lightly doped n-type.

A gate dielectric layer 286 overlies channel zone 266 of IGFET 220. Agate electrode 288 is situated on gate electrode 286 above channel zone266. Gate electrode 288 extends partially over each lateral S/Dextension 262E or 264E. In the example of FIG. 29, gate electrode 288consists of very heavily doped p-type polysilicon. A pair ofelectrically insulating sidewalls spacers 290 and 292 are situated alongthe opposite transverse sidewalls of p++ gate electrode 288. Metalsilicide layers 294, 296, and 298 are respectively situated along thetops of source 262, drain 264, and gate electrode 288.

Symmetric n-channel IGFET 230 has a pair of n-type S/D zones 302 and 304separated by a p-type channel zone 306 of p-type body material 308consisting of lower p− portion 114, a heavily doped immediate wellportion 316, and an upper portion 318. S/D zones 302 and 304 and channelzone 306 are situated in island 206. Each n-type S/D zone 302 or 304consists of a very heavily doped main portion 302M or 304M and a heavilydoped, and thus more lightly doped, lateral extension 302E or 304E forreducing drain-side hot-carrier injection. N+ lateral extensions 302Eand 304E terminate channel zone 306 along the upper semiconductorsurface.

A pair of heavily doped halo pocket portions 320 and 322 of p-type upperbody-material portion 318 respectively extend along S/D zones 302 and304 in a symmetric manner. P+ halo pocket portions 320 and 322 extendprimarily along n+ S/D extensions 302E and 304E. In the example of FIG.29, p+ pocket portions 320 and 322 extend deeper below the uppersemiconductor surface than n+ extensions 302E and 304E but not as deepas n++ main drain portions 302M and 304M. Item 324 is the moderatelydoped p-type remainder of upper body-material portion 318.

A gate dielectric layer 326 overlies channel zone 306. A gate electrode328 is situated on gate dielectric layer 326 above channel zone 306.Gate electrode 328 extends partially over each lateral S/D extension302E or 304E. Gate electrode 328 consists of very heavily doped n-typepolysilicon in the example of FIG. 29. A pair of electrically insulatingsidewalls spacers 330 and 332 are situated along the opposite transversesidewalls of n++ gate electrode 328. Metal silicide layers 334, 336, and338 are respectively situated along the tops of S/D zones 302 and 304and gate electrode 328.

Subject to being formed over p− lower body-material portion 214,symmetric p-channel IGFET 240 is a long-channel device configuredsubstantially the same as IGFET 230 with the conductivity typesreversed. IGFET 240 thus has a pair of p-type S/D zones 342 and 344separated by an n-type channel zone 346 of n-type body-material 348consisting of a heavily doped well portion 356 and an upper portion 358.S/D zones 342 and 344 and channel zone 346 are situated in island 208.Each p-type S/D zone 342 or 344 consists of a very heavily doped mainportion 342M or 344M and a more lightly doped, but still heavily doped,lateral extension 342E or 344E for reducing drain-side hot-carrierinjection. P+ lateral extensions 342E and 344E terminate channel zone346 along the upper semiconductor surface.

A pair of heavily doped halo pocket portions 360 and 362 of n-type upperbody-material portion 358 respectively extend along S/D zones 342 and344 in a symmetric manner. N+ halo pocket portions 360 and 362respectively extend primarily along S/D extensions 342E and 344E. In theexample of FIG. 29, n+ pocket portions 360 and 362 extend deeper belowthe upper semiconductor surface than n+ extensions 342E and 344E but notas deep as n++ main S/D portions 342M and 344M. Item 364 is themoderately doped n-type remainder of upper body-material portion 358.

A gate dielectric layer 366 overlies channel zone 346. A gate electrode368 is situated on gate dielectric layer 366 above channel zone 266.Gate electrode 288 extends partially over each S/D extension 342E or344E. In the example of FIG. 29, gate electrode 368 consists of veryheavily doped p-type polysilicon. A pair of electrically insulatingsidewall spacers 370 and 372 are situated along the opposite transversesidewalls of p++ gate electrode 368. Metal silicide layers 374, 376, and378 are respectively situated along the tops of S/D zones 342 and 344and gate electrode 368.

Gate dielectric layers 126, 286, 326, and 366 of IGFETs 210, 220, 230,and 240 typically consist primarily of silicon oxide but may consist ofsilicon oxynitride or/and other high permittivity dielectric material.The thickness of dielectric layers 126, 286, 326, and 346 is normally2-8 nm, preferably 3-5 nm, typically 3.5 nm for operation across a 1.8-Vrange. The dielectric layer thickness is suitably increased foroperation across a higher voltage range or suitably decreased foroperation across a lower voltage range. Sidewall spacers 250, 252, 290,292, 330, 332, 370, and 372 are illustrated in FIG. 29 as shaped roughlylike right triangles with convex hypotenuses but can have other shapes.Silicide layers 254, 256, 258, 294, 296, 298, 334, 336, 338, 374, 376,and 378 typically consist of cobalt silicide.

Channel zones 306 and 346 of IGFETs 230 and 240 have symmetriclongitudinal dopant profiles similar to that illustrated in FIG. 2 forsymmetric IGFET 20 of FIG. 1. The presence of p+ halo pocket portions320 and 322 in channel zone 306 alleviates threshold voltage roll-offand helps avoid punchthrough in IGFET 230. The presence of n+ halopocket portions 360 and 362 in channel zone 346 similarly alleviatesthreshold voltage roll-off and helps avoid punchthrough in IGFET 240.

The vertical dopant profile through each n++ main S/D portion 302M or304M of IGFET 230 and into underlying p-type body material 308 issimilar to that shown in FIG. 3 a for IGFET 20 and also to thecomputer-simulated reference dopant profiles shown in FIGS. 40, 44 a,and 44 b discussed below. The same applies to the vertical dopantprofile through each p++ main S/D portion 342M or 344M of IGFET 240 andinto underlying n-type body material 348 subject to body material 348forming a pn junction with p− lower portion 114 instead of merging intounderlying lightly doped n-type monosilicon. The moderate, but elevated,concentration of the p-type dopant in upper body-material portion 324 ofIGFET 230 cooperates with the heavy p-type dopant concentration providedby halo pocket portions 320 and 32 to inhibit punchthrough fromoccurring in IGFET 230. The corresponding dopings in upper body-materialportion 364 and halo pocket portions 360 and 362 of IGFET 240 similarlyenable it to avoid punchthrough.

The p-type well dopant that defines well portion 316 of IGFET 230typically reaches a maximum concentration at approximately the samedepth below the upper semiconductor surface as the maximum concentrationof the p-type well dopant that defines well portion 116 of IGFET 210.Because concentration N_(I) of the p-type background dopant isrelatively uniform, the maximum concentration of the total p-type dopantin well portion 316 of IGFET 230 typically occurs at approximately thesame depth below the upper surface as the maximum concentration of thetotal p-type dopant in well portion 116 of IGFET 210. Upperbody-material portion 318 of IGFET 230 is provided with p-typeanti-punchthrough (again “APT”) dopant to raise upper portion 318 to amoderate p-type doping level. The p-type APT dopant in upperbody-material portion 318 reaches a maximum concentration at a lesserdepth below the upper semiconductor surface than the maximumconcentration of the p-type well dopant of well portion 316.

The combination of the total p-type dopant, i.e., the p-type well, APT,and background dopants, in the portion of body material 308 below n++main S/D portion 302M or 304M causes concentration N_(T) of the totalp-type dopant in that body-material portion to be relatively flat alonga vertical line extending from the subsurface location of the maximump-type dopant concentration in well 316 up to main S/D portion 302M or304M. In particular, concentration N_(T) of the total p-type dopant inthe portion of body material 308 below main S/D portion 302M or 304Mnormally changes (decreases) by less than a factor of 10, typically byless than a factor of 5, in moving from the location of the maximump-type dopant concentration in well 316 up to portion 302M or 304M.

The same arises with IGFET 240. The n-type well dopant that defines wellportion 356 of IGFET 240 typically reaches a maximum concentration atapproximately the same depth below the upper semiconductor surface asthe maximum concentration of the n-type well dopant that defines wellportion 276 of IGFET 220. The maximum concentration of the total n-typedopant in well portion 356 of IGFET 240 thus typically occurs atapproximately the same depth below the upper surface as the maximumconcentration of the total n-type dopant in well portion 276 of IGFET220. Upper body-material portion 358 of IGFET 240 is provided withn-type APT dopant to raise upper portion 358 to a moderate n-type dopinglevel. The n-type APT dopant in upper body-material portion 358 reachesa maximum concentration at a lesser depth below the upper semiconductorsurface than the maximum concentration of the n-type well dopant of wellportion 356.

The combination of the total n-type dopant, i.e., primarily the p-typewell and APT dopants, in the portion of body material 348 below S/D zone342M or 344M causes concentration N_(T) of the total n-type dopant inthat body-material portion to be relatively flat along a vertical lineextending from the subsurface location of the maximum n-type dopantconcentration in well portion 356 up to main S/D portion 342M or 344M.Specifically, concentration N_(T) of the total n-type dopant in theportion of body material 348 below main S/D portion 342M or 344Mnormally changes by less than a factor of 10, typically by less than afactor of 5, in moving from the location of the maximum n-type dopantconcentration in well 356 up to portion 342M or 344M.

Halo pocket portions 120 and 280 of respective IGFETs 210 and 220 may,of course, alternatively extend deeper below the upper semiconductorsurface than respective sources 102 and 262. IGFET 210 then implementsIGFET 150 of FIG. 13 while IGFET 220 implements a p-channel version ofIGFET 150V. Halo pocket portions 320 and 322 of n-channel IGFET 230 mayextend deeper below the upper semiconductor surface than S/D zones 300and 302 as occurs below in computer-simulated reference short-channelstructure B of FIG. 40. Halo pocket portions 360 and 362 of p-channelIGFET 240 may likewise extend deeper below the upper semiconductorsurface than S/D zones 340 and 342.

FIGS. 30.1 and 30.2 (collectively “FIG. 30”) depict two portions ofanother complementary-IGFET semiconductor structure configured accordingto the invention so as to be particularly suitable for mixed-signalapplications. The complementary-IGFET structure of FIG. 30 containsasymmetric p-channel IGFET 220 and an asymmetric long n-channel IGFET380 configured the same as asymmetric n-channel IGFET 210 except that aheavily doped n-type subsurface layer 382 lies between p+ well portion116 and p− lower portion 114 for isolating p+ well portion 116 andp-type upper body-material portion 118 from p− portion 114.Consequently, p-type body material 108 for IGFET 380 does not include p−lower portion 114 but instead consists only of p+ well portion 116 andp-type upper body-material portion 118.

The complementary-IGFET structure of FIG. 30 further includes symmetricp-channel IGFET 240 and a symmetric long n-channel IGFET 390 configuredthe same as symmetric re-channel IGFET 230 except that a heavily dopedn-type subsurface layer 392 lies between p+ well portion 316 and p−lower portion 114 for isolating p+ well portion 316 and p-type upperbody-material portion 318 from p− portion 114. P-type body material 308for IGFET 390 thereby does not include p− lower portion 114 but insteadconsists only of p+ well portion 316 and p-type upper body-materialportion 318. Aside from n+ subsurface layers 382 and 392, n-channelIGFETs 380 and 390 operate respectively the same as n-channel IGFETs 210and 230.

Circuit elements other than IGFETs 210, 220, 230, 240, 380, and 390 maybe provided in other parts (not shown) of the complementary-IGFETstructure of FIG. 29 or 30. For instance, short-channel versions ofIGFETs 210, 220, 230, 240, 380, and 390 may be present in eithercomplementary-IGFET structure. Bipolar transistors along with varioustypes of resistors, capacitors, and/or inductors may be provided in thecomplementary-IGFET structure of FIG. 29 or 30. Depending on thecharacteristics of the additional circuit elements, suitable electricalisolation is also provided in either complementary-IGFET structure forthe additional elements. IGFETs 240 and 230 or 390 can, of course, bedeleted in some purely analog complementary-IGFET applications.

Fabrication of Complementary-IGFET Structure Suitable for Mixed-SignalApplications

FIGS. 31 a-31 o, 31 p.1-31 r.1, and 31 p.2-31 r.2 (collectively “FIG.31”) illustrate a semiconductor process in accordance with the inventionfor manufacturing a complementary-IGFET semiconductor structurecontaining long-channel IGFETs 210, 220, 230, and 240 as generally shownin FIG. 29. The steps involved in the fabrication of IGFETs 210, 220,230, and 240 up through the stage just before the creation of gatesidewall spacers 250, 252, 290, 292, 330, 332, 370, and 372 are shown inFIGS. 31 a-31 o. FIGS. 31 p.1-31 r.1 illustrate the fabrication ofspacers 250, 252, 290, and 292 and later steps leading to IGFETs 210 and220 as depicted in FIG. 29.1. FIGS. 31 p.2-31 r.2 illustrate thefabrication of spacers 330, 332, 370, and 372 and later steps leading toIGFETs 230 and 240 as shown in FIG. 29.2.

Short-channel versions of IGFETs 210, 220, 230, and 240 may bemanufactured simultaneously according to the fabrication steps employedin manufacturing long-channel IGFETs 210, 220, 230, and 240. Theshort-channel IGFETs are of lesser channel length than long-channelIGFETs 210, 220, 230, and 240 but otherwise are of generally the sameintermediate IGFET appearances as shown in FIG. 31. The simultaneousfabrication of long-channel IGFETs 210, 220, 230, and 240 and theirshort-channel versions is implemented with masking plates (reticles)having patterns for both the long-channel and short-channel IGFETs.

Aside from the pocket (including halo pocket) ion implantation steps andthe source/drain extension ion implantation steps, all of the ionimplantation steps in the present fabrication process are performedroughly perpendicular to the lower semiconductor surface and thusroughly perpendicular to the upper semiconductor surface. Moreparticularly, all of the implantation steps except the pocket andsource/drain extension ion implantation steps are performed at a smallangle, typically 7°, to the vertical. This small deviation fromperpendicularity is used to avoid undesirable ion channeling effects.For simplicity, the small deviation from perpendicularity is notindicated in FIG. 31.

Unless otherwise indicated, the species of n-type dopant utilized ineach of the n-type ion implantations in the fabrication process of FIG.31 consists of the specified n-type dopant in elemental form. That is,each n-type ion implantation is performed with ions of the specifiedn-type dopant element rather than with ions of a chemical compound thatcontains the n-type dopant. The species of p-type dopant employed ineach of the p-type ion implantations variously consists of the p-typedopant, normally boron, in elemental or compound form. Hence, eachp-type ion implantation is normally performed with boron ions or withions of a boron-containing compound such as boron difluoride.

In some of the fabrication steps in FIG. 31, openings extend(substantially) through a photoresist mask above the activesemiconductor regions for two IGFETs. When the two IGFETs are formedlaterally adjacent to each other in the exemplary cross sections of FIG.31, the two photoresist openings are illustrated as a single opening inFIG. 31 even through they may be described below as separate openings.

The letter “P” at the end of a reference symbol appearing in thedrawings of FIG. 31 indicates a precursor to a region which is shown inFIG. 29 and which is identified there by the portion of the referencesymbol preceding “P”. The letter “P” is dropped from the referencesymbol in the drawings of FIG. 31 when the precursor has evolvedsufficiently to largely constitute the corresponding region in FIG. 29.

The starting point for the fabrication process of FIG. 31 is amonosilicon semiconductor body typically consisting of a heavily dopedp-type substrate 400 and an overlying lightly doped p-type epitaxiallayer 114P. See FIG. 31 a. P+ substrate 400 is a semiconductor waferformed with <100> monosilicon doped with boron to a concentration ofapproximately 5×10¹⁸ atoms/cm³ for achieving a typical resistivity of0.015 ohm-cm. For simplicity, substrate 400 is not shown in theremainder of FIG. 31. Alternatively, the starting point can simply be ap-type substrate lightly doped substantially the same as p− epitaxiallayer 114P.

Epitaxial layer 114P consists of epitaxially grown <100> monosiliconlightly doped p type with boron to a concentration of approximately5×10¹⁵ atoms/cm³ for achieving a typical resistivity of 5 ohm-cm. Thethickness of epitaxial layer 114P is typically 5.5 μm. When the startingpoint for the fabrication process of FIG. 31 is a lightly doped p-typesubstrate, item 114P is the p− substrate.

Field-insulating region 200 is provided along the upper surface of p−epitaxial layer (or p-substrate) 114P as shown in FIG. 31 b so as todefine active semiconductor islands 202, 204, 206, and 208 respectivelyfor IGFETs 210, 220, 230, and 240 going from left to right in FIG. 31 b.Field insulation 200 is preferably created according to a trench-oxidetechnique but can be created according to a local-oxidation technique.In providing field insulation 200, a thin screen insulating layer 402 ofsilicon oxide is thermally grown along the upper surface of epitaxiallayer 114P.

A photoresist mask 404 having openings above islands 202 and 206 isformed on screen oxide layer 402 as shown in FIG. 31 c. P-type welldopant consisting of a boron species is ion implanted at a heavy dosageand a high energy through the uncovered sections of screen oxide 402 andinto the underlying monosilicon to define (a) p+ well portion 116 forIGFET 210 and (b) a p+ precursor well portion 316P for IGFET 230. Theportion of epitaxial layer 114P above well portion 116 constitutes a p−precursor upper body-material portion 118P for IGFET 210. Photoresist404 is removed.

A photoresist mask 406 having an opening above island 206 is formed onscreen oxide 402. See FIG. 31 d. P-type APT dopant consisting of a boronspecies is ion implanted at a moderate dosage through the uncoveredsection of screen oxide 402 and into the underlying monosilicon todefine a p precursor upper body-material portion 324P for IGFET 230.Photoresist 406 is removed.

A photoresist mask 408 having openings above islands 204 and 208 isformed on screen oxide 402 as shown in FIG. 31 e. N-type well dopantconsisting of phosphorus or arsenic is ion implanted at a heavy dosageand a high energy through the uncovered sections of screen oxide 402 andinto the underlying monosilicon to define (a) n+ well portion 276 forIGFET 220 and (b) an n+ precursor well portion 356P for IGFET 240.

With photoresist mask 408 in place, n-type compensating dopant likewiseconsisting of phosphorus or arsenic is ion implanted at a light dosageand a moderate energy through the uncovered section of oxide 402 aboveisland 204 and into the underlying monosilicon to define an n-precursorupper body-material portion 278P for IGFET 220. N− precursorbody-material portion 278P overlies n+ well portion 276 as it exists atthe stage of FIG. 31 e. The dosage and implant energy of the n-typecompensating dopant implant are normally sufficient to cause all ofprecursor body-material portion 278P to be of n-type conductivity.

The n-type compensating dopant also passes through the uncovered sectionof oxide 402 above island 208 and into the underlying monosilicon forIGFET 240. Either of the two n-type doping operations that utilizephotoresist 408 can be performed first. Photoresist 408 is removed. Ifit is desired that the monosilicon for IGFET 240 not receive any of then-type compensating dopant, the n-type compensating doping operation canbe performed with an additional photoresist mask having an opening aboveisland 204 but not above island 208 (and also not above islands 202 and206) after which the additional photoresist is removed.

During subsequent fabrication steps, some of the n-type well dopant usedto define precursor n+ well portion 276 for IGFET 220 diffuses upwardinto the semiconductor material above n+ well portion 276 as it existsat this point in the fabrication process of FIG. 29. That is, part ofthe n-type well dopant subsequently diffuses upward into overlyingmaterial of island 204 initially doped lightly p-type. The upwarddiffusion of part of the n-type well dopant that defines n+ well portion276 occurs primarily during subsequent fabrication steps performed atelevated temperature, i.e., temperature significantly greater than roomtemperature.

Depending on various factors, primarily the summation effects of (a) thetimes at the elevated temperatures of subsequent fabrication steps and(b) temperature parameters that favor increased diffusion of dopantswith those elevated temperatures, the upward-diffused part of the n-typewell dopant that defines n+ well portion 276 may becomes distributedthroughout island 204 for IGFET 220 in such a manner as to counterdopeall of the p-type dopant now in island 204. Ignoring any other dopantssubsequently introduced into island 204, this upward-diffused part ofthe n-type well dopant may thus cause all of island 204 to be convertedto n-type conductivity. In that case, the step of implanting the n-typecompensating dopant can sometimes be deleted to simplify the fabricationprocess and reduce manufacturing cost. FIGS. 32 a-32 c and 33 a-33 c,discussed below, describe two variations of the fabrication process ofFIG. 31 in which the step of implanting the n-type compensating dopantis deleted.

During subsequent fabrication, some of the n-type well dopant used todefine n+ well portion 356P for IGFET 240 also diffuses upward into thesemiconductor material above n+ well portion 356P as it exists at thispoint in the fabrication process of FIG. 29. However, as discussedbelow, all of island 208 for IGFET 240 is of n-type conductivity at theend of the ion implantation, and associated activation, of the n-typeAPT dopant introduced into island 208. Consequently, the decision toretain or delete the n-type compensating implant is determined by theconditions of the subsequent fabrication steps applied to island 204 forIGFET 220.

A photoresist mask 410 having an opening above island 208 is formed onscreen oxide 402. See FIG. 31 f. N-type APT dopant consisting ofphosphorus or arsenic is ion implanted at a moderate dosage through theuncovered section of screen oxide 402 and into the underlyingmonosilicon to define an n precursor upper body-material portion 358Pfor IGFET 240. Photoresist 410 is removed.

A thermal anneal, such as a rapid thermal anneal (“RTA”), may now beperformed on the resultant semiconductor structure to repair latticedamage and place the implanted p-type and n-type dopants inenergetically more stable states. See FIG. 31 g. The upper semiconductorsurface is cleaned. A gate-dielectric-containing dielectric layer 412 isprovided along the upper semiconductor surface as shown in FIG. 31 h.Dielectric layer 412 is created by a thermal growth technique.

Precursor gate electrodes 128P, 288P, 328P, and 368P are formed ongate-dielectric-containing dielectric layer 412 respectively abovesegments of upper body-material portions 118P, 278P, 318P, and 358P. SeeFIG. 31 i. Precursor gate electrodes 128P, 288P, 328P, and 368P arecreated by depositing a layer of largely undoped (intrinsic) polysiliconon dielectric layer 412 and then patterning the polysilicon layer. Theportions of dielectric layer 412 underlying precursor gate electrodes128P, 288P, 328P, and 368P respectively constitute gate dielectriclayers 126, 286, 326, and 366. The gate dielectric material formed withgate dielectric layers 126, 286, 326, and 366 generally respectivelyseparates gate electrodes 128P, 288P, 328P, and 368P from thebody-material segments intended to be respective channel zones 106, 266,306, and 346.

A dielectric sealing layer 414 is thermally grown along the exposedsurfaces of precursor gate electrodes 128P, 288P, 328P, and 368P. Againsee FIG. 31 i. In the course of forming dielectric sealing layer 414,the portions of dielectric layer 412 situated to the sides of gatedielectric layers 126, 286, 326, and 366 thicken somewhat to becomecomposite surface dielectric layer 416.

A photoresist mask 418 having an opening generally above the intendedlocation for p+ pocket portion 120 of IGFET 210 is formed on dielectriclayers 414 and 416. See FIG. 31 j. Photoresist mask 418 is criticallyaligned to precursor gate electrode 128P. P-type pocket dopantconsisting of a boron species is ion implanted at a moderate dosage inan angled manner through the uncovered portion of surface oxide layer416 and into the underlying monosilicon to define a p+ precursor pocketportion 120P for IGFET 210. The p-type pocket implantation is normallyperformed at two opposite tilt angles to the vertical. Alternatively,the p-type pocket implantation can be performed at a single tilt angle.Photoresist 418 is removed.

A photoresist mask 420 having an opening generally above the intendedlocation for n+ pocket portion 280 of IGFET 220 is formed on dielectriclayers 414 and 416. See FIG. 31 k. Photoresist mask 420 is criticallyaligned to precursor gate electrode 288P. N-type pocket dopantconsisting of phosphorus or arsenic is ion implanted at a heavy dosagein an angled manner through the uncovered portion of surface dielectric416 and into the underlying monosilicon to define an n+ precursor pocketportion 280P for IGFET 220. The n-type pocket implantation is normallyperformed at two opposite tilt angles but can be done at a single tiltangle. Photoresist 420 is removed.

A photoresist mask 422 having openings above islands 204 and 206 isformed on dielectric layers 414 and 416 as shown in FIG. 311. N-typesource/drain extension dopant consisting of arsenic or phosphorus is ionimplanted at a heavy dosage through the uncovered section of surfacedielectric 416 and into the underlying monosilicon to define (a) an n+precursor source extension 102EP for IGFET 210, (b) a separate n+precursor drain extension 104EP for IGFET 210, and (c) a pair oflaterally separated n+ precursor S/D extensions 302EP and 304EP forIGFET 230. Photoresist 422 is removed.

A photoresist mask 424 having an opening above island 206 is formed ondielectric layers 414 and 416. See FIG. 31 m. P-type halo dopantconsisting of a boron species is ion implanted at a heavy dosage in anangled manner through the uncovered section of surface dielectric 416and into the underlying monosilicon to define a pair of laterallyseparated p-type precursor halo pocket portions 320P and 322P for IGFET230. Photoresist 424 is removed.

A photoresist mask 426 having openings above islands 204 and 208 isformed on dielectric layers 414 and 416 as shown in FIG. 31 n. P-typesource/drain extension dopant consisting of a boron species is ionimplanted at a heavy dosage through the uncovered section of surfaceoxide 416 and into the underlying monosilicon to define (a) a p+precursor source extension 262EP for IGFET 220, (b) a separate p+precursor drain extension 264EP for IGFET 220, and (c) a pair oflaterally separated p+ S/D extensions 342EP and 344EP for IGFET 240.Photoresist 426 is removed.

A photoresist mask 428 having an opening above island 208 is formed ondielectric layers 414 and 416. See FIG. 31 o. N+ halo dopant consistingof phosphorus or arsenic is ion implanted at a heavy dosage in an angledmanner through the uncovered section of surface dielectric 416 and intothe underlying monosilicon to define a pair of laterally separated n+precursor halo pocket portions 360P and 362P for IGFET 240. Photoresist428 is removed.

A low-temperature furnace anneal may be performed at this point toremove defects caused by the heavy dosages of the source/drain extensionimplants.

In the remainder of the process of FIG. 31, the complementary-IGFETstructure at each processing stage is illustrated with a pair of FIGS.“31 z.1” and “31 z.2” where “z” is a letter varying from “p” to “r”.Each FIG. 31 z.1 illustrates the processing done to create asymmetricIGFETs 210 and 220 while each FIG. 31 z.2 illustrates the processingsimultaneously done to create symmetric IGFETs 230 and 240. Each pair ofFIGS. 31 z.1 and 31 z.2 is, for convenience, collectively referred tobelow as “FIG. 31 z” where “z” varies from “p” to “r”. For instance,FIGS. 31 p.1 and 31 p.2 are collectively referred to as “FIG. 31 p”.

Gate sidewall spacers 250, 252, 290, 292, 330, 332, 370, and 372 areformed along the transverse sidewalls of precursor gate electrodes 128P,288P, 328P, and 368P as shown in FIG. 31 p. The formation of sidewallspacers 250, 252, 290, 292, 330, 332, 370, and 372 is performed bydepositing dielectric material on top of the structure and thenremoving, primarily by anisotropic etching conducted generallyperpendicular to the upper semiconductor surface, the dielectricmaterial not intended to constitute spacers 250, 252, 290, 292, 330,332, 370, and 372. Portions of dielectric layers 414 and 416 are alsopartially, but not totally, removed. Items 430 and 432 in FIG. 31 prespectively indicate the remainders of dielectric layers 414 and 416not covered by spacers 250, 252, 290, 292, 330, 332, 370 and 372.

A photoresist mask 434 having openings above islands 204 and 206 isformed on dielectric layers 430 and 432 and spacers 290, 292, 370, and372. See FIG. 31 q. N-type main source/drain dopant consisting ofarsenic or antimony is ion implanted at a very heavy dosage through theuncovered sections of surface dielectric layer 432 and into theunderlying monosilicon to define (a) n++ main source portion 102M andn++ main drain portion 104M for IGFET 210 and (b) n++ main S/D portions302M and 304M for IGFET 230. The n-type main source/drain dopant alsoenters precursor electrodes 128P and 328P and converts them respectivelyinto n++ gate electrodes 128 and 328. Photoresist 434 is removed.

The portions of regions 102EP, 104EP, and 120P outside main S/D portions102M and 104M now respectively constitute n+ source extension 102E, n+drain extension 104E, and p+ pocket portion 120 for IGFET 210. P− upperbody-material remainder 124 is the remaining lightly doped material ofprecursor upper body-material portion 118P, now p-type upperbody-material portion 118. The portions of regions 302EP, 304EP, 320P,and 322P outside main S/D portions 302M and 304M now respectivelyconstitute n+ S/D extensions 302E and 304E and p+ halo pocket portions320 and 322 for IGFET 230. P− upper body-material remainder 324 is theremaining lightly doped p-type material of precursor upper body-materialportion 318P, now p-type upper body-material portion 318.

When the main source/drain dopant consists of arsenic, a thermal annealmay now be performed to repair lattice damage, activate the main n-typesource/drain dopant, and cause it to diffuse outward. This anneal,normally an RTA, also activates the pocket and source/drain extensiondopants.

A photoresist mask 436 having openings above islands 204 and 208 isformed on dielectric layers 430 and 432 and spacers 250, 252, 330, and332 as shown in FIG. 31 r. P-type main source/drain dopant consisting ofa boron species is ion implanted at a very heavy dosage through theuncovered section of surface dielectric 432 and into the underlyingmonosilicon to define (a) p++ main source portion 262M and p++ maindrain portion 264M for IGFET 220 and (b) p++ main S/D portions 342M and344M for IGFET 240. The p-type main source/drain dopant also entersprecursor electrodes 288P and 368P and converts them respectively intop++ gate electrodes 288 and 368. Photoresist 436 is removed.

The portions of regions 262EP, 264EP, and 280P outside main S/D portions262M and 264M now respectively constitute p+ source extension 262E, p+drain extension 264E, and n+ pocket portion 280 for IGFET 220. N− upperbody-material remainder 284 is the remaining lightly doped n-typematerial of n− upper body-material portion 278P, now n-type upperbody-material portion 278. The portions of regions 342EP, 344EP, 360P,and 362P outside main S/D portions 342M and 344M now respectivelyconstitute p+ S/D extensions 342E and 344E and n+ halo pocket portions360 and 362 for IGFET 240. N− upper body-material remainder 364 is theremaining lightly doped n-type material of n− precursor upperbody-material portion 358P, now n-type upper body-material portion 358.

A capping layer (not shown) of dielectric material, typically siliconoxide, is formed on top of the structure. The semiconductor structure isthen thermally annealed to repair lattice damage and activate theimplanted main p-type source/drain dopant. If the earlier anneal foractivating the main n-type source/drain dopant is not performed, thisfinal anneal activates the pocket dopants and all the source/draindopants. The final anneal is typically an RTA.

The thin layers of dielectric material, including dielectric layers 430and 432, are removed along the upper semiconductor surface and along thetop surfaces of gate electrodes 128, 288, 328, and 368. Metal silicidelayers 254, 256, 258, 294, 296, 298, 334, 336, 338, 374, 376, and 378are respectively formed along the upper surfaces of regions 102M, 104M,128, 262M, 264M, 288, 302M, 304M, 328, 342M, 344M, and 368. Thistypically entails depositing a thin layer of suitable metal, typicallycobalt, on the upper surface of the structure and performing alow-temperature step to react the metal with underlying silicon Theunreacted metal is removed. A second low-temperature step is performedto complete the reaction of the metal with the underlying silicon andthereby form silicide layers 254, 256, 258, 294, 296, 298, 334, 336,338, 374, 376, and 378. The metal silicide formation completes the basicfabrication of IGFETs 210, 220, 230, and 240. The resultantcomplementary-IGFET structure appears as shown in FIG. 29.

The p-type well, p-type APT, n-type well, n-type compensating, andn-type APT implantations of FIGS. 31 c-31 f can generally be performedin any order. The p-type pocket, n-type pocket, n-type source/drainextension, p-type halo, p-type source/drain extension, and n-type haloimplantations of FIGS. 31 j-31 o can generally be performed in anyorder. The n-type main source/drain implantation of FIG. 31 q isnormally performed before the p-type main source/drain implantation ofFIG. 31 r, particularly when the main source/drain dopant consists ofarsenic.

However, the p-type main source/drain implantation can sometimes beperformed before the n-type main source/drain implantation.

The tilt angles for the p-type pocket, n-type pocket, p-type halo, andn-type halo implantations of FIGS. 31 j, 31 k, 31 m, and 31 o arenormally at least 15°. Although typically varying from one of the angledimplantations to another, the tilt angle for each angled implantation istypically 25-45°.

The complementary-IGFET structure of FIG. 30 is, except for n+ isolationlayers 382 and 392 that convert IGFETs 210 and 230 respectively intoIGFETs 380 and 390, typically fabricated according to substantially thesame steps as the complementary-IGFET structure of FIG. 29. Isolationlayers 382 and 392 are normally formed between the stages of FIGS. 31 band 31 c using an additional photoresist mask having openings aboveislands 202 and 206. The additional photoresist mask also has openingsfor use in creating heavily doped n-type regions that connect isolationlayers 382 and 392 to the upper semiconductor surface for receiving asuitable isolation voltage. Isolation dopant consisting of arsenic orphosphorus is ion implanted at a heavy dosage through the uncoveredsections of screen oxide 402 and into the underlying monosilicon todefine (a) n+ isolation layers 382 and 392 respectively for IGFETs 380and 390 and (b) the n+ isolation-layer connection regions.

The fabrication process of FIG. 31 can be modified as described below tochange asymmetric n-channel IGFET 210 from an implementation of IGFET180 to an implementation of asymmetric n-channel IGFET 190 of FIG. 18 cin which n-type S/D zones 102 and 104 respectively further include n+lower S/D portions 102L and 104L that respectively underlie n++ main S/Dportions 102M and 104M. Because lower S/D portions 102L and 104L aremore lightly doped n-type than main S/D portions 102M and 104M, lowerS/D portions 102L and 104L provide source/drain vertical dopant gradingto further reduce source/drain parasitic capacitances as described abovefor IGFET 160 of FIG. 15.

This process modification begins at the stage of FIG. 31 q withphotoresist mask 434 in place for use in ion implanting n++ main S/Dportions 102M and 104M. N-type lower source/drain dopant consisting ofphosphorus or arsenic is ion implanted at a heavy dosage through theuncovered sections of surface dielectric layer 432 and into theunderlying monosilicon to define n+ lower S/D portions 102L and 104L.The implantation of n+ lower S/D portions 102L and 104L can be performedbefore or after the implantation of n++ main S/D portions 102M and 104M.

The implantation energies for the n-type main and lower source/draindopants are chosen such that the n-type lower source/drain dopant is ofgreater implantation range than the n-type main source/drain dopant.Inasmuch as both the n-type main source/drain implantation and then-type lower source/drain implantation are performed solely throughsurface dielectric layer 432, the n-type lower source/drain dopant isimplanted to a greater average depth below the upper semiconductorsurface than the n-type main source/drain dopant With the n-type mainsource/drain dopant being implanted at a very heavy dosage and thus at agreater dosage than the n-type lower source/drain dopant, n+ lower S/Dportions 102L and 104L are more lightly doped than, and extend deeperbelow the upper semiconductor surface than, n++ main S/D portions 102Mand 104M.

Symmetric n-channel IGFET 230 is simultaneously converted into avariation in which n-type S/D zones 302 and 304 respectively furtherinclude a pair of lower S/D portions more lightly doped than main S/Dportions 302M and 304M. As with n+ lower S/D portions 102L and 104L forthe preceding variation of IGFET 210, the lower S/D portions for thevariation of IGFET 230 are heavily doped n-type. If it is desired thatIGFET 230 not be converted into a variation having the n+ lower S/Dportions, the implantation of the n-type lower source/drain dopant canbe performed with an additional photoresist mask having an opening aboveisland 202 but not above island 206 (and also not above islands 204 and208) after which the additional photoresist is removed.

The fabrication process of FIG. 31 can be similarly modified to changeasymmetric p-channel IGFET 220 from an implementation of a p-channelversion of n-channel IGFET 180V to an implementation of a p-channelversion of n-channel IGFET 190V for which p-type S/D zones 262 and 264respectively further include a pair of heavily doped p-type lower S/Dportions that respectively underlie p++ main S/D portions 262M and 264M.The p+ lower S/D portions provide source/drain vertical dopant gradingto further reduce source/drain parasitic capacitances analogous to thatdescribed above for IGFET 160 of FIG. 15.

This additional process modification begins at the stage of FIG. 31 rwith photoresist mask 436 in place for use in ion implanting p++ mainS/D portions 262M and 264M. P-type lower source/drain dopant consistingof a boron species is ion implanted at a heavy dosage through theuncovered sections of surface dielectric layer 432 and into theunderlying monosilicon to define the two p+ lower S/D portions for thevariation of IGFET 220. The n+ lower source/drain implantation can beperformed before or after the n++ main source/drain implantation. In oneexample, the p-type lower source/drain dopant consists of elementalboron while the p-type main source/drain dopant consists of borondifluoride.

The implantation energies for the p-type main and lower source/draindopants are chosen such that the p-type lower source/drain dopant is ofgreater implantation range than the p-type main source/drain dopant.With both the p-type main source/drain implantation and the p-type lowersource/drain implantation being performed solely through surfacedielectric layer 432, the p-type lower source/drain dopant is implantedto a greater average depth below the upper semiconductor surface thanthe p-type main source/drain dopant Since the p-type main source/draindopant is implanted at a very heavy dosage and thus a greater dosagethan the p-type lower source/drain dopant, the p+ lower S/D portions forthe variation of IGFET 220 are more lightly doped than, and extenddeeper below the upper semiconductor surface than, p++ main S/D portions262M and 264M.

Symmetric p-channel IGFET 240 is simultaneously converted into avariation in which p-type S/D zones 362 and 364 respectively furtherinclude a pair of lower S/D portions more lightly doped than main S/Dportions 362M and 3604M. As with the p+ lower S/D portions for thevariation of IGFET 220, the lower S/D portions for the variation ofIGFET 240 are heavily doped p-type. If IGFET 240 is not to be convertedinto a variation having the lower S/D portions, the p-type lowersource/drain implantation can be performed with an additionalphotoresist mask having an opening above island 204 but not above island208 (and also not above islands 202 and 206) after which the additionalphotoresist is removed.

Fabrication Process Variations that Avoid N-Type Compensating Implant

FIGS. 32 a-32 c (collectively “FIG. 32”) illustrate an alternative, inaccordance with the invention, to the step of FIG. 31 e formanufacturing a variation of the complementary-IGFET semiconductorstructure of FIG. 29. The use of an n-type compensating implant intoisland 204 (and island 208) is avoided in the fabrication process ofFIG. 31 as modified to incorporate the alternative of FIG. 32. As aresult, the complementary-IGFET structure fabricated by using thealternative of FIG. 32 contains a variation 220V of asymmetric p-channelIGFET 220.

The process alternative of FIG. 32 begins with the structure of FIG. 31d repeated here as FIG. 32 a. The n-type well doping step describedabove in connection with FIG. 31 e is performed on the structure of FIG.32 a. In particular, photoresist mask 408 is formed on screen oxide 402as shown in FIG. 32 b. Photoresist mask 408 again has openings aboveislands 204 and 208. The n-type well dopant, again consisting ofphosphorus or arsenic, is ion implanted at a heavy dosage and a highenergy through the uncovered sections of screen oxide 402 and into theunderlying monosilicon to define (a) a precursor n+ well portion 276Pfor asymmetric p-channel IGFET 220V and (b) n+ precursor well portion356P for symmetric p-channel IGFET 240.

The n-type compensating implant into island 204 (and island 208) withphotoresist 408 in place is not performed at this point. Instead,photoresist 408 is simply removed. After removal of photoresist 408, alightly doped p-type portion 278Q of island 204 for IGFET 220V ispresent above precursor n+ well portion 276P. A lightly doped p-typeportion 358Q of island 208 for IGFET 240 is similarly present aboveprecursor n+ well portion 356P.

The alternative of FIG. 32 continues with the formation of photoresistmask 410 on screen oxide 402. See FIG. 32 c. Photoresist 410 again hasan opening above island 208. The n-type APT dopant, again consisting ofphosphorus or arsenic, is ion implanted at a moderate dosage through theuncovered section of screen oxide 402 and into the underlyingmonosilicon to define n precursor upper body-material portion 358P forIGFET 240. Photoresist 410 is removed.

The n-type APT dopant implanted into island 208 converts all of p−portion 358Q of island 208 to n-type conductivity. Consequently, p−portion 358Q disappears. Inasmuch as no significant amount of the n-typeAPT dopant enters island 204, p− portion 278Q of island 204 is stillpresent at this stage of fabrication.

The structure of FIG. 32 c is further processed according to thefabrication steps described above in connection with FIGS. 31 g-31 o, 31p.1-31 r.1, and 31 p.2-31 r.2, including the associated annealingoperations. Some of these further steps are performed at elevatedtemperatures, again temperatures significantly greater than roomtemperature. During the elevated-temperature steps, part of the n-typewell dopant used to define precursor n+ well portion 276P for IGFET 220Vdiffuses upward into p− portion 278Q. The upward-diffused part of then-type well dopant becomes distributed throughout island 204 in such amanner that all of its p− material not significantly subjected to p-typeor/and n-type doping subsequent to the n-type well doping step isconverted to n-type conductivity by the end of the fabrication. Subjectto being doped n-type somewhat more lightly than n-type upperbody-material portion 278 fabricated fully according to the basicfabrication process of FIG. 31, p− portion 278Q largely becomes n-typeupper body-material portion 278 in the complementary-IGFET structuremade according to the fabrication process of FIG. 31 as modified toemploy the alternative of FIG. 32. N− upper body-material remainder 284is again the remaining lightly doped n-type material of n-typebody-material portion 278.

The complementary-IGFET structure made according to the fabricationprocess of FIG. 31 as modified to incorporate the alternative of FIG. 32appears largely as shown in FIG. 29. A generalized version of asymmetricp-channel IGFET 220V made by using the alternative of FIG. 32 isdepicted in FIG. 34 discussed below.

FIGS. 33 a-33 f (collectively “FIG. 33”) illustrate an alternative, inaccordance with the invention, to the steps of FIGS. 31 c-31 f formanufacturing a variation of the complementary-IGFET semiconductorstructure of FIG. 29. As with the alternative of FIG. 32, thealternative of FIG. 33 avoids the use of an n-type compensating implantinto island 204 (and island 208). Consequently, the complementary-IGFETstructure fabricated according to the process of FIG. 31 as modified toincorporate the alternative of FIG. 33 contains asymmetric p-channelIGFET 220V in place of IGFET 220.

The process alternative of FIG. 33 begins with the structure of FIG. 31b repeated as FIG. 33 a. Screen oxide layer 402 has been formed alongthe upper surface of epitaxial layer 114P at the stage of FIG. 33 a.However, no ion implantation into any of islands 204, 204, 206, and 208has yet been made.

Photoresist mask 408 having openings above islands 204 and 208 is formedon screen oxide 402 as shown in FIG. 33 b. The n-type well dopant, onceagain consisting of phosphorus or arsenic, is ion implanted at a heavydosage and a high energy through the uncovered sections of screen oxide402 and into the underlying monosilicon to define (a) precursor n+ wellportion 276P for IGFET 220V and (b) n+ precursor well portion 356P forIGFET 240. Photoresist 408 is removed. After removal of photoresist 408,p− portion 278Q of island 204 for IGFET 220V is present above precursorn+ well portion 276P. P− portion 358Q of island 208 for IGFET 240 issimilarly present above precursor n+ well portion 356P.

A thermal anneal, preferably an RTA, is normally performed on theresultant semiconductor structure at this point to repair lattice damageand place the atoms of the implanted n-type well dopant in energeticallymore stable states. See FIG. 33 c. During the anneal, part of the n-typewell dopant used to define precursor n+ well portion 276P for IGFET 220Vdiffuses upward into p− portion 278Q. Part of the n-type well dopantused to define precursor n+ well portion 356P for IGFET 240 similarlydiffuses upward into p− portion 358Q. The upward diffusion of theseparts of the n-type well dopant is typically sufficient to convertrespective lower parts of p− portions 278Q and 358Q to n-typeconductivity. The so-converted lower parts of p− portions 278Q and 358Qare respectively labeled as precursor n− upper body-material portions278P and 358P in FIG. 33 c. Due to the formation of precursor n−body-material portions 278P and 358P, p− portions 278Q and 358Q shrinkvertically in size as generally indicated in FIG. 33 c.

As described further below, more of the n-type well dopant used todefine precursor n+ well portion 276P diffuses upward into p− portion278Q during later steps in the fabrication process of FIG. 31 asmodified to employ the alternative of FIG. 33. Similar to what occurs inthe alternative of FIG. 32, the total upward-diffused part of the n-typewell dopant in the alternative of FIG. 33 becomes distributed throughoutisland 204 in such a manner that all of its p− material notsignificantly subjected to p-type or/and n-type doping subsequent to then-type well doping step is converted to n-type conductivity by the endof the fabrication. Importantly, the partial upward diffusion of then-type well dopant to define precursor n− upper body-material portion278P as it exists at the stage of FIG. 33 c occurs without affecting thep-type well dopant, the n-type APT dopant, the p-type APT dopant, or anyof the source/drain dopants because the steps for introducing thosedopants into the semiconductor structure have not yet been performed.Hence, performing the n-type well implant at this point in thefabrication process helps ensure that all of the p− material of island204 not subjected to other (later) p-type or/and n-type doping iseventually converted to n-type conductivity without causing undesirablediffusion of the p-type well dopant, the n-type APT dopant, the p-typeAPT dopant, or any of the source/drain dopants.

Photoresist mask 410 is formed on screen oxide 402. See FIG. 33 d.Photoresist 410 once again has an opening above island 208. The n-typeAPT dopant, which consists of phosphorus or arsenic, is ion implanted ata moderate dosage through the uncovered section of screen oxide 402 andinto the underlying monosilicon to define precursor upper body-materialportion 358P for IGFET 240. The n-type APT dopant implanted into island208 converts all of p− portion 358Q of island 208 to n-typeconductivity, thereby causing p− portion 358Q to disappear. Photoresist410 is removed.

Photoresist mask 404 is formed on screen oxide layer 402 as shown inFIG. 33 e. Photoresist 404 again has openings above islands 202 and 206.The p-type well dopant, which consists of a boron species, is ionimplanted at a heavy dosage and a high energy through the uncoveredsections of screen oxide 402 and into the underlying monosilicon todefine (a) p+ well portion 116 for IGFET 210 and (b) p+ precursor wellportion 316P for IGFET 230. The portion of epitaxial layer 114P abovewell portion 116 again constitutes p− precursor upper body-materialportion 118P for IGFET 210. Photoresist 404 is removed.

Photoresist mask 406 is formed on screen oxide 402. See FIG. 33 f.Photoresist 406 again has an opening above island 206. The p-type APTdopant, which consists of a boron species, is ion implanted at amoderate dosage through the uncovered section of screen oxide 402 andinto the underlying monosilicon to define p precursor upperbody-material portion 324P for IGFET 230. Photoresist 406 is removed.

The structure of FIG. 33 f is further processed according to thefabrication steps described above in connection with FIGS. 31 g-31 o, 31p.1-31 r.1, and 31 p.2-31 r.2, including the associated annealingoperations. During later elevated-temperature steps, more of the n-typewell dopant used to define precursor n+ well portion 276P for IGFET 220Vdiffuses upward into p− portion 278Q of island 204 until all of theisland's p− material not subjected to other p-type or/and n-type dopingduring the fabrication process is converted to n-type conductivity.Subject to being doped n-type somewhat more lightly than n-type upperbody-material portion 278 fabricated fully according to the basicfabrication process of FIG. 31, p− portion 278Q largely becomes n-typeupper body-material portion 278 in the complementary-IGFET structuremade according to the fabrication process of FIG. 31 as modified toutilize the alternative of FIG. 33. N− upper body-material remainder 284once again is the remaining lightly doped n-type material of n-typebody-material portion 278.

The complementary-IGFET structure made according to the fabricationprocess of FIG. 31 using the alternative of FIG. 33 appears largely asshown in FIG. 29. The p-channel IGFET depicted in FIG. 34 discussedbelow is also a generalized version of IGFET 220V made by using thealternative of FIG. 33.

The semiconductor portions of p-channel IGFET 220V are, as describedabove, created from the lightly doped p-type material of island 204. Inorder to assure that all of the island's p-material not subjected top-type or/and n-type doping other than the n-type well doping during thefabrication process is converted to n-type conductivity by the end ofthe fabrication, the concentration of the n-type well dopant along theupper surface of island 204 at the end of the fabrication must exceedthe initial concentration of the p-type dopant in island 204. Sinceisland 204 is formed from part of p− epitaxial layer 114P (or a p-typesubstrate lightly doped substantially the same as epitaxial layer 114P),the upper-surface concentration of the n-type well dopant in island 204at the end of fabrication must exceed the p-type background dopantconcentration in epitaxial layer 114P.

The doping and thermal processing conditions in one implementation ofthe fabrication process of FIG. 31 as modified to incorporate thealternative of FIG. 32 or 33 are chosen such that the concentration ofthe n-type well dopant along the upper surface of island 204 at the endof fabrication is expected to be at least twice the p-type backgrounddopant concentration in epitaxial layer 114P. Choosing the doping andthermal processing conditions in this way makes it highly probable thatthe upper-surface concentration of the n-type well dopant in island 204at the end of fabrication will actually exceed the p-type backgrounddopant concentration in epitaxial layer 114P in view of typicalfabrication process variations. So choosing the doping and thermalprocessing conditions may entail changing the p-type background dopantconcentration in epitaxial layer 114P from what is described aboveor/and changing the n-type well doping conditions from what is describedabove. Such changes are further dealt with below in connection withFIGS. 35 a-35 c, 36 a-36 c, 37 a-37 c, and 38 a-38 c.

Channel zone 266 of p-channel IGFET 220V is normally asymmetricallylongitudinally dopant graded similar to, and typically somewhat stronger(greater) than, that described above for p-channel IGFET 220. Subject toreversal of the conductivity types, the asymmetric longitudinal gradingin channel zone 266 of IGFET 220V is thus similar to, and typicallysomewhat stronger than, that described above for IGFETs 180V and 150V.

P-channel IGFET 220V typically has a hypoabrupt vertical dopant profilebelow drain 264 similar to, but somewhat weaker than, that describedabove for p-channel IGFET 220. In particular, the concentration of thetotal n-type dopant in the portion of body material 268 below drain 264of IGFET 220V decreases by at least a factor of 10, typically a factorin the vicinity of 15, in moving from the subsurface location of themaximum concentration of the n-type dopant in well portion 276vertically up to drain 264 with the location of the maximumconcentration of the n-type dopant in well portion 276 being no morethan 10 times, normally no more than 5 times, deeper below the uppersemiconductor surface than drain 264. Again subject to reversal of theconductivity types, the hypoabrupt vertical dopant profile below drain262 of IGFET 220V is therefore typically similar to, but somewhat weakerthan, that described above for n-channel IGFETs 180V and 100V. For thereasons discussed below in connection with FIGS. 38 a-38 c, thehypoabrupt vertical dopant profile below drain 264 of IGFET 220V enablesit to have increased analog speed.

In an alternative process embodiment, the p-type background dopantconcentration in epitaxial layer 114P, the n-type well dopingconditions, and the subsequent thermal processing conditions areadjusted such that the concentration of the total n-type dopant in theportion of body material 268 below drain 264 of IGFET 220V decreases byless than a factor of 10 in moving from the subsurface location of themaximum concentration of the n-type dopant in well portion 276vertically up to drain 264. Although the resulting analog speed may notbe as great as when the vertical dopant profile below drain 264 of IGFET220V is hypoabrupt, fabricating IGFET 220 V according to this processembodiment still simplifies the fabrication process and reducesmanufacturing cost.

Asymmetric p-channel IGFET 220V can replace IGFET 220 in thecomplementary-IGFET semiconductor structure of FIG. 30. Except for n+isolation layers 382 and 392 that convert IGFETs 210 and 230respectively into IGFETs 380 and 390, the so-modified version of thecomplementary-IGFET structure of FIG. 30 is typically fabricatedsubstantially according to the process of FIG. 31 as modified toincorporate the alternative of FIG. 32 or 33. In employing thealternative of FIG. 32, isolation layers 382 and 392 are again normallyformed between the stages of FIGS. 31 b and 31 c using theabove-mentioned additional photoresist mask having openings aboveislands 202 and 206. When the alternative of FIG. 33 is employed,isolation layers 382 and 392 are formed between the stages of FIGS. 33 aand 33 b in the same way as in the alternative of FIG. 32. Isolationdopant diffusion which occurs during the thermal anneal performeddirectly after the n-type well implant in using the alternative of FIG.33 normally has substantially no damaging effect on the resultantcomplementary-IGFET structure.

In using the alternative of FIG. 32 or 33, the additional photoresistmask again also has openings for use in creating heavily doped n-typeregions that connect isolation layers 382 and 392 to the uppersemiconductor surface for receiving the isolation voltage. The isolationdopant, which consists of arsenic or phosphorus, is ion implanted at aheavy dosage through the uncovered sections of screen oxide 402 and intothe underlying monosilicon to define (a) n+ isolation layers 382 and 392respectively for IGFETs 380 and 390 and (b) the n+ isolation-layerconnection regions.

P-Channel IGFETs Having Hypoabrupt Vertical Body-Material Dopant ProfileBelow Drain Due to Subsurface Maximum in Well Dopant Concentration butAvoiding N-Type Compensating Implant

FIG. 34 illustrates, in accordance with the invention, a generalizedversion 220U of asymmetric p-channel IGFET 220V in which n+ well portion276 is of opposite conductivity type to directly underlying p−semiconductor-material portion 114. Asymmetric p-channel IGFET 220U isof IGFET characteristics that arise from being fabricated according tothe invention without using the compensatory n-type dopant implantationinto the p-type portion of island 204 situated directly above n+ wellportion 276 as initially defined by ion implantation of the n-type welldopant. In essence, IGFET 220V fabricated according to the process ofFIG. 31 as modified to incorporate the alternative of FIG. 32 or 33 isan implementation of IGFET 220U.

P-channel IGFET 220U consists of two-part p-type source 262, two-partp-type drain 264, n-type body material 268, gate dielectric layer 286,and gate electrode 288. N-type body material 268 is again formed with n+well portion 276 and n-type upper body-material portion 278 consistingof n+ source-side pocket portion 280 and an n-type body-materialremainder 394. N-type channel zone 266 of n-type upper body-materialportion 278 similarly again laterally separates n-type S/D zones 262 and264. Components 262, 264, 266, 268, 276, 278, 280, 286, and 288 of IGFET220U are configured and doped largely the same as in IGFET 220V.

Item 396 in FIG. 34 indicates the pn junction between source 262 andbody material 268. Item 398 indicates the pn junction between drain 264and body material 268. Analogous to the re-channel IGFETs, items y_(S)and y_(D) indicate the depths to which source 262 and drain 264 ofp-channel IGFET 220U respectively extend below the upper semiconductorsurface.

N-type upper body-material remainder 394 of IGFET 220U is, forsimplicity, labeled “n-” in FIG. 34 analogous to how n-type upperbody-material remainder 284 is identified by the label “n-” herein. Infabricating IGFET 220U according to the process of FIG. 31 as modifiedto incorporate the alternative of FIG. 32 or 33, upper body-materialremainder 394 normally receives n-type dopant substantially only byupdiffusion from underlying n+ well portion 276. As explained furtherbelow in connection with FIGS. 37 a-37 c, the n-type dopantconcentration in body-material remainder 394 normally progressivelydecreases in going from well portion 276 up to the upper semiconductorsurface. Since well portion 276 is heavily doped n-type, body-materialremainder can alternatively be pictorially viewed as consisting of alightly doped n-type surface-adjoining part and a moderately dopedn-type intermediate part situated between n+ well portion 276 and thelightly doped n-type surface-adjoining part.

An understanding of the doping characteristics of IGFET 220U isfacilitated with the assistance of FIGS. 35 a-35 c (collectively “FIG.35”), FIGS. 36 a-36 c (collectively “FIG. 36”), FIGS. 37 a-37 c(collectively “FIG. 37”), and FIGS. 38 a-38 c (collectively “FIG. 38”).FIG. 35 presents exemplary dopant concentrations along the uppersemiconductor surface as a function of longitudinal distance x.Exemplary dopant concentrations as a function of depth y along avertical line 130U through source 262 are presented in FIG. 36. FIG. 37presents exemplary dopant concentrations as a function of depth y alonga pair of vertical lines 132U and 134U through channel zone 266.Vertical line 132U passes through source-side pocket portion 280.Vertical line 134 passes through a vertical location between pocketportion 280 and drain 264. Exemplary dopant concentrations as a functionof depth y along a vertical line 136U through drain 264 are presented inFIG. 38. Vertical lines 130U, 132U, 134U, and 136U for p-channel IGFET220U respectively correspond to vertical lines 130, 132, 134, and 136for the n-channel IGFETS of the invention.

FIG. 35 a illustrates concentrations N_(I), along the uppersemiconductor surface, of the individual semiconductor dopants thatlargely define regions 262, 264, 280, and 394 and thus establish thelongitudinal dopant grading of channel zone 266. FIGS. 36 a, 37 a, and38 a illustrate concentration N_(I), along vertical lines 130U, 132U,134U, and 136U, of the individual semiconductor dopants that verticallydefine regions 114, 262, 264, 276, 280, and 394 and thus establish ahypoabrupt vertical dopant profile in the portion of body material 268underlying drain 264. Curves 262′ and 264′ represent concentrationsN_(I) (surface and vertical) of the p-type dopant used to respectivelyform source 262 and drain 264. Curves 276′, 280′, and 394′ representconcentrations N_(I) (surface and/or vertical,) of the n-type dopantsused to respectively form regions 276, 280, and 394. Items 396 ^(#) and398 ^(#) indicate where net dopant concentration N_(N) goes to zero andthus respectively indicate the locations of pn junctions 396 and 398.

Concentrations N_(T) of the total p-type and total n-type dopants inregions 262, 264, 280, and 394 along the upper semiconductor surface areshown in FIG. 35 b. FIGS. 36 b, 37 b, and 38 b depict, along verticallines 130U, 132U, 134U, and 136U, concentrations N_(T) of the totalp-type and total n-type dopants in regions 114, 262, 264, 276, 280, and394. Curve segments 276″, 280″, and 394″ respectively corresponding toregions 276, 280, and 394 represent total concentrations N_(T) of then-type dopant. Item 266″ in FIG. 35 b corresponds to channel zone 266and represents the channel-zone portions of curve segments 280″ and394″. Total concentrations N_(T) of the p-type dopant are represented bycurves 262″ and 264″ respectively corresponding to source 262 and drain264.

FIG. 35 c illustrates net dopant concentration N_(N) along the uppersemiconductor surface. Net dopant concentration N_(N) along verticallines 130U, 132U, 134U, and 136U is presented in FIGS. 36 c, 37 c, and38 c. Curve segments 276*, 280*, and 394* represent net concentrationsN_(N) of the n-type dopant in respective regions 276, 280, and 394. Item266* in FIG. 35 c represents the combination of channel-zone curvesegments 280* and 394* and thus presents concentration N_(N) of the netn-type dopant in channel zone 266. Concentrations N_(N) of the netp-type dopant in source 262 and drain 264 are respectively representedby curves 262* and 264*.

The upper-surface dopant distributions shown in FIG. 35 for p-channelIGFET 220U are respectively substantially the same as the upper-surfacedopant distributions shown in FIG. 26 for re-channel IGFET 180V exceptthat (a) the background p-type dopant concentration for p− lower portion114, as indicated by curve 114′ in FIG. 35 a, is less than thebackground n-type dopant concentration for n− lower portion 192, asindicated by items 192′ in FIG. 26 a, and (b) the upper-surface dopantconcentration for n− upper body-material remainder 394, as indicated byitems 394′, 394″, and 394* in FIG. 35, is less than the upper-surfacedopant concentration for p− upper body-material remainder 198, asindicated by items 198′, 198″, and 198* in FIG. 26. P-channel IGFET 220Uis, nonetheless, configured similarly to n-channel IGFET 180V. Hence,the comments made above about the upper-surface dopant distributionsshown in FIG. 26 for n-channel IGFET 180V apply largely to theupper-surface dopant distributions shown in FIG. 35 for p-channel IGFET220U subject to changing regions 102, 102M, 102E, 104, 104M, 104E, 106,120, 192, and 198 for IGFET 180V respectively to regions 262, 262M,262E, 264, 264M, 264E, 266, 280, 114, and 394 for IGFET 220U and subjectto the indicated dopant concentration differences.

Next consider FIG. 37. The n-type dopant in n+ pocket portion 280consists of (a) a major portion constituted by the n-type pocket dopant,indicated by dopant concentration curve 280′ in FIG. 37 a, and (b) aminor portion constituted by an updiffused part of the n-type welldopant, indicated by dopant concentration curve 276′ in FIG. 37 a. Then-type dopant in n− upper body-material remainder 394 consistssubstantially solely of an updiffused part of the n-type well dopant.Concentration N_(I) of the updiffused part of the n-type well dopant inn− body-material remainder 394 is indicated by portion 394′ of n-typewell dopant concentration curve 276′ in FIG. 37 a. Portions 394″ and394* of respective dopant concentration curves 276″ and 276* in FIGS. 37b and 37 c similarly respectively indicate total dopant concentrationN_(T) and net dopant concentration N_(N) in n-body-material remainder394.

With source-side pocket portion 280 of IGFET 220U being heavily dopedn-type and with the n-type doping of n− upper body-material remainder394 being provided substantially only by updiffusion from n+ wellportion 276, the lowest n-type dopant concentration in channel zone 266occurs in n− body-material remainder 394 along or close to its uppersurface depending on whether any significant amount of the updiffusedportion of the n-type well dopant piles up along the upper surface ofremainder 394. FIG. 37 illustrates an example in which there is nosignificant n-type well dopant pile-up along the upper surface of n−body-material remainder 394.

As mentioned above in connection with the fabrication of p-channel IGFET220V whose semiconductor portions are created in island 204 formed frompart of p− epitaxial layer 114P (or a p-type substrate lightly dopedsubstantially the same as epitaxial layer 114P), the concentration ofthe n-type well dopant along the upper surface of island 204 at the endof fabrication must exceed the p-type background dopant concentration inepitaxial layer 114P. Upon being applied to IGFET 220U, this fabricationdoping requirement converts into the structural doping requirement thatconcentration N_(I) of the n-type well dopant along the upper surface ofchannel zone 266, specifically the upper surface of n− upperbody-material remainder 394, exceed concentration N_(I) of the p-typebackground dopant in p-lower portion 114. The desirable fabricationobjective that the doping and thermal processing conditions be chosensuch that the upper-surface concentration of the n-type well dopant inisland 204 at the end of fabrication of IGFET 220V is expected to be atleast twice the p-type background dopant concentration in epitaxiallayer 114P similarly converts into the structural doping requirementthat concentration N_(I) of the n-type well dopant along the uppersurface of n-body-material remainder 394 be at least twice concentrationN_(I) of the p-type background dopant.

FIGS. 35-38 depict an example in which n-type well dopant concentrationN_(I), as indicated by curve segment 394′ in FIGS. 35 a and 37 a, alongthe upper surface of n-body-material remainder 394 is approximatelytwice p-type background dopant concentration N_(I), as indicated bycurve 114′ in FIGS. 35 a and 37 a, of p− lower region 114. Hence, theexample of FIGS. 35-38 satisfies the specific structural dopingrequirement that concentration N_(I) of the n-type well dopant along theupper surface of n− body-material remainder 394 be at least twiceconcentration N_(I) of the p-type background dopant.

Concentration N_(I) of the p-type background dopant is approximately1×10¹⁶ atoms/cm³ in the example of FIGS. 35-38. This is the lower limitof the range of 1×10¹⁵-1×10¹⁶ atoms/cm³ given above for the p-typebackground dopant concentration. However, in using the alternative ofFIG. 32 or 33 to implement IGFET 220U as IGFET 220V, the lower limit ofthe range for the p-type background dopant concentration is shifteddownward to 5×10¹⁴ atoms/cm³ or less. For island 204 used to createIGFET 220V, this provides more flexibility in assuring that that all ofthe island's p-material not subjected to p-type or/and n-type dopingother than the n-type well doping during the fabrication process isconverted to n-type conductivity by the end of fabrication.

Concentration N_(I) of the n-type well dopant in IGFET 220U reaches amaximum value at depth y_(W) below the upper semiconductor surface. Thetotal n-type dopant in the portions of body material 268 below source262 and drain 264 of IGFET 220U consists solely of the n-type welldopant as indicated by curve 276′ in FIGS. 36 a and 38 a. Consequently,concentration N_(T) of the total n-type dopant in the portions of p-bodymaterial 268 below source 262 and drain 264 of IGFET 220U reaches amaximum value at depth y_(W) as indicated by curve 276″ in FIGS. 36 band 38 b.

As shown by the variation in curve 276″ in FIG. 38 b, concentrationN_(T) of the total n-type dopant in the portion of body material 268below drain 264 of IGFET 220U decreases by at least a factor of 10,typically a factor in the vicinity of 15, in moving from the subsurfacelocation of the maximum concentration of the n-type dopant in wellportion 276 along vertical line 136U up to drain 264. Also, the locationof the maximum concentration of the total n-type dopant in well portion276 is no more than 10 times, normally no more than 5 times, deeperbelow the upper semiconductor surface than drain 264. Hence, thevertical dopant profile below drain 264 of IGFET 220U is hypoabrupt.Furthermore, concentration N_(T) of the total n-type dopant in theportion of body material 268 below drain 264 of IGFET 220U normallydecreases progressively in moving from the location of the maximumn-type well concentration up to drain 264 as indicated by the portion ofcurve 276′ extending from depth y_(W) of the maximum concentration ofthe total n-type dopant in the portion of body material 268 below drain264 up to item 398# representing drain-body junction 398.

The net dopant in the portion of body material 268 below drain 264 ofIGFET 220U is n-type dopant. FIG. 38 c shows that, as represented bycurve 276*, concentration N_(N) of the net dopant in the portion of bodymaterial 268 below drain 264 of IGFET 220U varies vertically in asimilar manner to concentration N_(T) of the total n-type dopant in theportion of body material 268 below drain 264 except that concentrationN_(N) in the portion of body material 268 below drain 264 drops to zeroat drain-body junction 398. The hypoabrupt vertical dopant profile inthe portion of body material 268 below drain 264 of IGFET 220U causesthe parasitic capacitance associated with drain-body junction 398 todecrease for the reasons discussed above in connection with p-channelIGFET 220 and the asymmetric n-channel IGFETs of the invention. Althoughthe hypoabrupt vertical dopant profile in the portion of body material268 below drain 264 is not as strong in IGFET 220U, which avoids then-type compensating implant, as in IGFET 220, which uses the n-typecompensating implant, the hypoabrupt vertical dopant profile in theportion of body material 268 below drain 264 of IGFET 220U causesdrain-body junction 398 to have reduced parasitic capacitance. IGFET220U thereby has increased analog speed.

Concentrations N_(I) and N_(T) of the total n-type dopant in the portionof body material 268 below source 262 of IGFET 220U vary verticallylargely the same as concentrations N_(I) and N_(T) of the total n-typedopant in the portion of body material 268 below drain 264. Comparecurves 276′ and 276″ in FIGS. 36 a and 36 b taken along vertical line130U through source 262 to curves 276′ and 276″ in FIGS. 38 a and 38 btaken along vertical line 136U through drain 264. Taking note of thefact that the net dopant in the portion of body material 268 belowsource 262 of IGFET 220U is n-type dopant, concentration N_(N) of thenet dopant in the portion of body material 268 below source 262 variesvertically largely the same as concentration N_(N) of the net dopant inthe portion of body material 268 below drain 264. As represented bycurve 276* in FIG. 36 c, concentration N_(N) of the net dopant in theportion of body material 268 below source 262 of IGFET 220U thereforevaries vertically in a similar manner to concentration N_(T) of thetotal n-type dopant in the portion of body material 268 below source 262except that concentration N_(N) in the portion of body material 268below source 262 drops to zero at source-body junction 396. Thisvertical dopant profile below source 262 of IGFET 220U normally causesthe parasitic capacitance along source-body junction 396 to be reduced.

General Computer Simulations

Computer simulations were conducted to check the device characteristicsand performance advantages, especially for analog applications, ofIGFETs configured according to the invention. The simulations wereperformed with (a) the MicroTec two-dimensional device simulatorfurnished by Siborg Systems and (b) the Medici two-dimensional devicesimulator furnished by Avant! Corp. The MicroTec simulator was utilizedprimarily for large-signal (DC) simulations. The Medici simulator wasemployed primarily for small-signal simulations.

Two types of n-channel IGFETs were computer simulated at the devicelevel: (a) asymmetric n-channel IGFETs configured according to theinvention and (b) symmetric reference n-channel IGFETs generallycorresponding to (but different inventively from) the computer-simulatedinventive n-channel IGFETs. The inventive computer-simulated asymmetricIGFETs are generally indicated below as being of structure “A”. Thecomputer-simulated inventive asymmetric IGFETs of structure A generallycorrespond to long n-channel IGFET 150 of FIG. 13 or to a short-channelversion of IGFET 150. The reference computer-simulated symmetric IGFETsare generally identified below as being of structure “B”. Thecomputer-simulated reference IGFETs of structure B generally correspondto long n-channel IGFET 230 of FIG. 29 or to a short-channel version ofIGFET 230 for the situation in which halo pocket portions 320 and 322 ofIGFET 230 extend respectively below S/D zones 302 and 304.

Structures A and B were based on analytical-profile models generallyusing Gaussian dopant profiles. It was assumed that each set ofstructures A and B under comparison was manufactured according to thesame process flow such as that of FIG. 31. Aside from variations thatproduce the body-material doping features of the invention and except asotherwise indicated below, structures A and B in each set undercomparison had substantially the same dopant distributions. Structures Aand B in each set under comparison also had basically the samegeometrical dimensions, e.g., gate length, gate stack height, andsource/drain length. The computer simulations on structures A and Brepresented devices manufactured at the 0.18-μm technology node, i.e.,IGFETs manufactured with design rules for which the minimum printablefeature size was 0.18 μm.

The computer simulations were generally directed toward enhancing analogperformance. Structure B was thus computer simulated at parameter valuesexpected to yield enhanced analog performance for structure A. Inasmuchas the basic architecture of structure B is for digital applications,the values for certain of the parameters used in computer simulatingstructure B to achieve enhanced analog performance differed from theparameter values that would yield enhanced digital performance.

It was assumed that the structures A and B were arranged inmultiple-IGFET structures and that insulation-filled trenches were usedto implement a field-insulating region, such as field-insulating region200 in FIG. 29, for laterally isolating the IGFETs in the multiple-IGFETstructures. The trenches, which had the same dimensions for structures Aand B, were up to 50% deeper and up to 20% wider than that needed tooptimize digital performance for reference structure B in themultiple-IGFET structures. The IGFET wells, corresponding to p+ wellportions 116 and 316 respectively for asymmetric n-channel IGFET 150 inFIG. 13 and symmetric n-channel IGFET 230 in FIG. 29, were up to 20%deeper than that needed to optimize the digital performance of structureB. It appears that inventive structure A may need the wider/deepertrenches in order to maintain good inter-IGFET isolation in the presenceof a well whose average dopant concentration is lower than that ofstructure B optimized for digital performance.

Structures A and B both had the same threshold voltage V_(T) and thesame background p-type dopant concentration, namely 0.4 V and 5×10¹⁵atoms/cm³. For a given value of the p-type background dopantconcentration, the p-type implants which define the pocket portionsalong the upper semiconductor surfaces of structures A and B controltheir threshold voltages V_(T). In light of this, the peak upper-surfaceconcentration of the p-type pocket dopant in the single pocket portionof inventive structure A was appropriately adjusted to achieve the samethreshold voltage that arose from the peak upper-surface concentrationsof the p-type halo dopant in the two halo pocket portions of referencestructure B. More specifically, inventive asymmetric structure Areceived a heavier dosage of the pocket dopant than reference symmetricstructure B to accommodate the fact that reference structure B had twiceas many pocket portions as inventive structure A. The higher doping inthe single pocket portion of inventive asymmetric structure A increasesits punchthrough resistance compared to an asymmetric IGFET structureidentical to structure A except that the pocket portion in thecomparison asymmetric structure has the same doping as either pocketportion of reference symmetric structure B.

FIG. 39 presents a three-dimensional dopant profile for animplementation of a short-channel version of inventive asymmetricstructure A. A three-dimensional dopant profile for an implementation ofa corresponding short-channel version of reference symmetric structure Bis presented in FIG. 40. FIGS. 39 and 40 specifically illustrate netdopant concentration N_(N) as a function of longitudinal distance x anddepth y. Only the monosilicon portions of the implementations ofstructures A and B are presented in FIGS. 39 and 40. Although both ofstructures A and B were assumed to be fabricated at the 0.18-μmtechnology node, the implementations of structures A and B in FIGS. 39and 40 of the computer simulations both had a gate length L_(G) of 0.2μm leading to a channel length L of 0.12-0.14 μm due to lithographyalignment.

The implementation of inventive structure A in FIG. 40 basicallycorresponds to a short-channel version of IGFET 150. The implementationof reference structure B in FIG. 40 basically corresponds to ashort-channel version of IGFET 230 subject to the implementation ofstructure B in FIG. 40 being simulated generally at the parameter valueswhich enhance the analog performance of the implementation of structureA and again subject to halo pocket portions 320 and 322 of IGFET 230extending respectively below S/D zones 302 and 304. For convenience, theimplementations of short-channel structures A and B in FIGS. 39 and 40are labeled with the reference symbols employed to identify themonosilicon regions of IGFETs 150 and 230. Since structure B wassimulated for analog applications, S/D zones 302 and 304 arerespectively labeled as source 302 and drain 304 in FIG. 40. Except asindicated below, all references to the short-channel versions ofstructures A and B, or to short-channel structures A and B, mean theimplementations respectively shown in FIGS. 39 and 40.

Each S/D zone of each structure A or B consisted of a very heavily dopedmain portion and a more lightly doped, but still heavily doped, lateralextension. Source 102 of the short-channel version of structure A thusconsisted of main portion 102M and lateral extension 102E. However,regions 102M and 102E of source 102 are difficult to distinguish in FIG.39 and are therefore not separately labeled in FIG. 39.

FIGS. 41 and 42 present two-dimensional dopant contours for therespective short-channel versions of structures A and B. The dopantcontours are taken along vertical planes through short-channelstructures A and B. Regions 104M and 104E of short-channel structure Acan be clearly distinguished in FIG. 41 and are so labeled there. Asshown by the locations of pn junctions 110 and 112 in FIG. 41, maindrain portion 104M of short-channel structure A extends deeper below theupper semiconductor surface than main source portion 102M. That is,drain depth y_(D) is greater than source depth y_(S) for inventiveasymmetric structure A. In contrast, an examination of FIG. 42 showsthat main source portion 302M and main drain portion 304M in referencesymmetric structure B extend to virtually the same depth below the uppersemiconductor surface.

FIG. 43 presents net dopant concentration N_(N) along the uppersemiconductor surface as a function of longitudinal distance x from ameasurement-reference S/D zone location for the short-channel versionsof structures A and B. The measurement-reference S/D zone location isapproximately 3.5 μm from the channel-zone center. In FIG. 43 and in thelater graphs which present computer-simulated data for both ofstructures A and B, curves representing data for inventive structure Aare marked with small empty circles to distinguish that data from thedata for reference structure B whose data curves are not additionallymarked. At locations where structures A and B have substantiallyidentical data, the curve segments for structures A and B are notvisually distinguishable from each other.

Analogous to the curve segments in FIG. 14 c, curve segments 102M*,104M*, 102E*, and 104E* in FIG. 43 represent concentrations N_(N) of thenet n-type dopant respectively in regions 102M, 104M, 102E, and 104Ealong the upper semiconductor surface of inventive short-channelstructure A. Curve segments 106* and 120* in FIG. 43 representconcentrations N_(N) of the net p-type dopant respectively in regions106 and 120 along the upper semiconductor surface of short-channelstructure A. Curve segments 302M*, 304M*, 302E*, and 304E* representconcentrations N_(N) of the net p-type dopant respectively in regions302M, 304M, 302E, and 304E along the upper semiconductor surface ofreference short-channel structure B. Curve segments 306*, 320*, and 322*represent concentrations of the net p-type dopant respectively inregions 306, 320, and 322 along the upper semiconductor surface ofshort-channel structure B.

Curve segment 106* in FIG. 43 illustrates the asymmetric dopant gradingin channel zone 106 of inventive short-channel structure A. Inparticular, curve segment 106* shows that net dopant concentration N_(N)along the upper semiconductor surface of short-channel structure Areaches a high value at approximately 1×10¹⁸ atoms/cm³ near source 102and then decreases progressively in moving from the location of thathigh value across channel zone 106 toward drain 104. Although not shownin the computer simulations, concentration N_(T) of the total p-typedopant in channel zone 106 of short-channel structure A is at least afactor of 10 lower, typically a factor of more than 100 lower, wherezone 106 meets source 102 along the upper surface than where zone 106meets drain 104 along the upper surface. In contrast, the symmetricdopant grading in channel zone 316 of short-channel structure B isillustrated by curve segment 306* which shows that concentration N_(N)along the upper surface of short-channel structure B is at approximatelyequal peak values close to both source 102 and drain 104 and is at aslightly lower value in the middle of channel zone 306.

FIG. 44 a presents absolute (total p-type and total n-type) verticaldopant profiles through S/D locations for the short-channel versions ofstructures A and B. Using the same measurement-reference S/D zonelocation from which longitudinal distance x is measured in FIG. 43,absolute dopant concentrations N_(T) for inventive short-channelstructure A are illustrated in FIG. 44 a along a vertical line (orplane) through main source portion 102M at distance x equal to 0.0 μmand along a vertical line through main drain portion 104M at distance xequal to 0.7 μm. FIG. 44 a similarly depicts concentrations N_(T) forreference short-channel structure B along a vertical line extendingthrough main source portion 302M at distance x equal to 0.0 μm and alonga vertical line extending through main drain portion 304M at distance xequal to 0.7 μm.

Analogous to the curve segments in FIGS. 8 b and 10 b, curve segments102″ and 104″ in FIG. 44 a respectively correspond to source 102 anddrain 104 of inventive short-channel structure A and thus representconcentrations N_(T) of the total n-type dopant along the vertical linesrespectively at distances x of 0.0 and 0.7 μm through source 102 anddrain 104, specifically main source portion 102M and main drain portion104M. Curve segments 114″, 116″, 118″, 120″, and 124″ in FIG. 44 arespectively generally correspond to regions 114, 116, 118, 120, and 124of short-channel structure A and thus variously represent concentrationsN_(T) of the total p-type dopant along the vertical lines at distances xof 0.0 and 0.7 μm through source 102 and drain 104. Items 110 ^(#) and112 ^(#) respectively indicate pn junctions 110 and 112 forshort-channel structure A.

The maximum concentration of the p-type well dopant in well portion 116of inventive short-channel structure A occurs at depth y_(W) equal to0.7 μm as indicated by curve segment 116″ in FIG. 44 a. The combinationof curve segments 116″ and 124″ illustrates the hypoabrupt nature of thevertical dopant profile below drain 104 along the vertical line atdistance x equal to 0.7 μm in short-channel structure A. In particular,combined curve segment 116″/124″ shows that concentration N_(T) of thetotal p-type dopant in the body material portion directly below drain104 decreases by a factor of approximately 100, and thus by a factorconsiderably in excess of 10, in moving from the location of the maximump-type well dopant concentration up to pn junction 112 at the bottom ofdrain 104.

Depth y_(D) of drain-body junction 112 is approximately 0.2 μm ininventive short-channel structure A dealt with in FIG. 44 a. Inasmuch asdepth y_(W) of maximum concentration N_(T) of the total p-type dopant inthe body material portion below drain 104 is 0.7 μm, the location ofmaximum concentration N_(T) of the total p-type dopant in the bodymaterial portion below drain 104 in short-channel structure A isapproximately 3.5 times deeper below the upper semiconductor surfacethan drain 104. Consequently, concentration N_(T) of the total p-typedopant in body material 108 decreases by a factor of approximately 100in moving upward to drain 104 from the maximum p-type well dopantconcentration location which is no more than 5 times deeper below theupper semiconductor surface than drain 104.

Curve segments 302″ and 304″ in FIG. 44 a respectively correspond tosource 302 and drain 304 of reference short-channel structure B andrepresent concentrations N_(T) of the total n-type dopant along thevertical lines at distances x of 0.0 and 0.7 μm through source 302 anddrain 304. Curve segments 114″, 316″, 318″, 320″, and 322″ respectivelycorrespond to regions 114, 316, 318, 320, and 322 of short-channelstructure B and thus represent concentrations N_(T) of the total p-typedopant along the vertical lines at distances x of 0.0 and 0.7 μm throughsource 302 and drain 304. In this regard, curve segment 114″ is utilizedfor both of short-channel structures A and B.

As is the situation with well portion 116 of inventive short-channelstructure A, the maximum concentration of the p-type well dopant in wellportion 316 of reference short-channel structure B occurs at depth y_(W)equal to 0.7 μm as indicated by curve segment 316″ in FIG. 44 a.However, the combination of curve segments 316″, 318″, and 322″ (or320″) is relatively flat in the p-type body-material portion directlybelow drain 304 (or source 302) in short-channel structure B. Combinedcurve segments 316″/318″/322″ shows that concentration N_(T) of thetotal p-type dopant in the body-material portion directly below drain304 changes considerably less than a factor of 5, and thus considerablyless than a factor of 10, in moving from the location of the maximump-type well dopant concentration up to the pn junction at the bottom ofdrain 304. That is, the vertical dopant profile in the body-materialportion directly below drain 304 in short-channel structure B is nothypoabrupt.

The net vertical dopant profiles corresponding to the absolute verticaldopant profiles of FIG. 44 a are presented in FIG. 44 b for theshort-channel versions of structures A and B. Curve segments 102* and104* in FIG. 44 b represent concentrations N_(N) of the net n-typedopant respectively in source 102 and drain 104 of inventiveshort-channel structure A along the vertical lines respectively atdistances x of 0.0 and 0.7 μm through source 102 and drain 104,specifically through main source portion 102M and main drain portion104M. Curve segments 114*, 116*, 120*, and 124* represent concentrationsN_(N) of the net p-type dopant respectively in regions 114, 116, 120,and 124 of short-channel structure A variously along the vertical linesat distances x of 0.0 and 0.7 μm through source 102 and drain 104.

Curve segments 302* and 304* in FIG. 44 b represent concentrations N_(N)of the net n-type dopant respectively in source 302 and drain 304 ofreference short-channel structure B along the vertical linesrespectively at distances x of 0.0 and 0.7 μm through source 302 anddrain 304, specifically through main source portion 302M and main drainportion 304M. Curve segments 114*, 316*, and 318* representconcentrations N_(N) of the net p-type dopant respectively in regions114, 316, and 318 of short-channel structure B along the vertical linesat distances x of 0.0 and 0.7 μm through source 302 and drain 304. Curvesegment 114* is used for both of short-channel structures A and B.

FIGS. 45 a and 45 b respectively illustrate lineal linear-rangetransconductance g_(mw) and lineal saturation transconductance g_(msatw)as a function of gate-to-source voltage V_(GS) for the short-channelversions of structures A and B for which gate length L_(G) was 0.2 μm.FIGS. 46 a and 46 b respectively illustrate lineal linear-rangetransconductance g_(mw) and lineal saturation transconductance g_(msatw)as a function of gate-to-source voltage V_(GS) for long-channel versionsof structures A and B configured basically the same as short-channelstructures A and B except that gate length L_(G) was 0.5 μm.Drain-to-source voltage V_(DS) was 0.1 V for the linear-range g_(mw)graphs of FIGS. 45 a and 46 a. Voltage V_(DS) was 2.0 V for thesaturation g_(msatw) graphs of FIGS. 45 b and 46 b. FIGS. 45 a, 45 b, 46a, and 46 b also indicate the variation of lineal drain current I_(Dw)from which linear-range transconductance g_(mw) and saturationtransconductance g_(msatw) were determined.

As FIGS. 46 a and 46 b show, the long-channel version of inventivestructure A exhibited considerably higher transconductance, bothlinear-range transconductance g_(mw) and saturation transconductanceg_(msatw), than the long-channel version of reference structure B. Theshort-channel version of structure A exhibited slightly higher, roughly10% higher, linear-range transconductance g_(mw) than the short-channelversion of structure B as indicated in FIG. 45 a. FIG. 45 b shows thatthe short-channel versions of structures A and B had nearly the sameg_(msatw) characteristics. The generally higher values oftransconductances g_(mw) and g_(msatw) for inventive structure A enableit to have higher voltage gain and thus improve its analog performance.

FIG. 47 illustrates the current-voltage transfer characteristics, i.e.,the variation of lineal drain current I_(Dw) with gate-to-source voltageV_(GS), for the short-channel versions of structures A and B at adrain-to-source voltage V_(DS) of 2.0 V. As FIG. 47 shows, short-channelstructures A and B had almost identical current-voltage characteristics.It is also expected that long-channel versions of structures A and Bwould have largely the same current-voltage characteristics.

Taking note of the fact that upper body-material portion 318 ofreference short-channel structure B is provided with a highconcentration of the p-type APT dopant for helping to avoid punchthroughin short-channel structure B, the nearly identical current-voltagecharacteristics for short-channel structures A and B show that theabsence, in inventive structure A, of the p-type APT dopant at thelocation generally similar to that of the p-type APT dopant in referencestructure B does not lead to punchthrough in inventive structure A. Thequalitative physical explanation for this result is that the p-typepocket dopant performs the anti-punchthrough function in structure A.More particularly, the p-type pocket implant in pocket portion 120 ofinventive structure A is provided at a greater doping than the p-typehalo implant in either of pocket portions 320 and 322 of referencestructure B in order for structure A to have the same threshold voltageV_(T) as structure B. This difference can be seen by comparing curvesegment 120* in FIG. 43 to curve segments 320* and 322*. The higherdoping of the p-type pocket implant in inventive structure A enables itto avoid punchthrough while simultaneously operating at low currentleakage comparable to that of reference structure B.

The preceding conclusion is further supported by computer-simulated dataobtained on a further reference short-channel IGFET structure C lackingthe APT implant of reference short-channel structure B but otherwiseidentical to short-channel structure B. The current-voltage transfercharacteristics for reference structure C at the V_(DS) value of 2.0 Vare indicated by the curve tagged C in FIG. 47. Leakage current I_(D0w),is the value of drain current I_(Dw) at a zero value of gate-to-sourcevoltage V_(GS). As FIG. 47 shows, drain leakage current I_(D0w) isapproximately 50 times higher for reference structure C than forinventive short-channel structure A. This indicates that punchthroughoccurs in reference structure C.

FIG. 48 depicts lineal drain current I_(Dw) as a function ofdrain-to-source voltage V_(DS) for the short-channel versions ofstructures A and B at values of gate-to-source voltage V_(GS) rangingfrom 0.5 V to 2.0 V. As indicated in FIG. 48, inventive short-channelstructure A generally achieves slightly higher values of drain currentI_(Dw) than reference short-channel structure B at each of the indicatedV_(GS) values. Inventive structure A thus has lower channel resistancethan reference structure B. Also, drain current I_(Dw) increases lesswith increasing drain-to-source voltage V_(DS) at high voltage V_(DS) ininventive structure A than in reference structure B. This indicates thatless avalanche multiplication or/and less channel-width modulationoccurs in inventive structure A than in reference structure B.

Analytical Analysis and Performance Advantages of Inventive AsymmetricIGFETs

For good analog performance, the source of an IGFET should be as shallowas reasonably possible in order to avoid roll-off of threshold voltageV_(T) at short-channel length. The source should also be doped asheavily as possible in order to the maximize the IGFET's effectivetransconductance g_(meff) in the presence of the source resistanceR_(S). Effective transconductance g_(meff) is determined from theIGFET's intrinsic transconductance g_(m) as:

$\begin{matrix}{g_{meff} = \frac{g_{m}}{1 + {R_{s}g_{m}}}} & (1)\end{matrix}$As Eq. 1 indicates, reducing source resistance R_(S) causes effectivetransconductance g_(meff) to increase. The voltage drop across sourceresistance R_(S) also subtracts from the intrinsic gate-to-sourcevoltage so that actual gate-to-source voltage V_(GS) is at a lowervalue. This debiases the IGFET at its gate electrode. In short, sourceresistance R_(S) should be as low as reasonably possible.

The need to minimize source resistance R_(S) in order to maximizeeffective transconductance g_(meff) is in addition to the need to have alower series resistance at the source and drain of an IGFET in order toachieve a lower of value of the IGFET's on-resistance R_(on). Moreparticularly, the voltage drop across source resistance R_(S) adds tothe total source-to-drain voltage drop. This causes on-resistance R_(on)to increase.

For achieving high-voltage capability and reducing hot-carrierinjection, the drain of an IGFET should be as deep and lightly doped asreasonably possible. These needs should be met without causingon-resistance R_(on) to increase significantly and without causingshort-channel threshold voltage roll-off.

The parasitic capacitances of an IGFET play an important role in settingthe speed performance of the circuit containing the IGFET, particularlyin small-signal high-frequency operations. FIG. 47 illustrates parasiticcapacitances C_(DB), C_(SB), C_(GB), C_(GD), and C_(GS) variouslyassociated with drain electrode D, source electrode E, gate electrode E,and body-region electrode B of an n-channel IGFET Q where C_(DB)represents the drain-to-body capacitance, C_(SB) represents thesource-to-body capacitance, C_(GB) represents the gate-to-bodycapacitance, C_(GD) represents the gate-to-drain capacitance, and C_(GS)represents the gate-to-source capacitance. A small-signal equivalentmodel of IGFET Q is presented in FIG. 50 where V_(BS) is thebody-to-source voltage, g_(mb) is the transconductance of the bodyelectrode, and items 440 and 442 are current sources.

The bandwidth of an amplifier is defined as the value of the frequencyat which the amplifier's gain drops to 1/√{square root over (2)}(approximately 0.707) of its low-frequency value. It is generallydesirable that the amplifier's bandwidth be as great as possible.

IGFET Q of FIG. 47 can be arranged in three primary amplifierconfigurations for providing amplified output voltage V_(out) as afunction of input voltage V_(in) according to the relationship:V_(out)=H_(A)V_(in)  (2)where H_(A) is the IGFET's complex transfer function. These threeconfigurations are the common-source, common-gate, and common-drainconfigurations respectively shown in FIGS. 51 a-51 c where C_(L) is theload capacitance, V_(DD) is the high supply voltage, and V_(SS) is thelow supply voltage. Amplifier input voltage V_(in) is supplied from avoltage source 444. Element 446 in FIGS. 51 b and 51 c is a currentsource, while signal V_(G) in FIG. 51 b is the gate voltage. Anexamination of transfer functions H_(A) for the three configurations ofFIGS. 51 a-51 c shows that reducing parasitic drain-to-body capacitanceC_(DB) and/or parasitic source-to-body capacitance C_(SB) improves IGFETperformance in each of these configurations.

Transfer function H_(A) for the common-source amplifier configuration ofFIG. 51 a is the input-pole/output-pole function:

$\begin{matrix}{H_{A} = \frac{{- g_{m}}R_{D}}{\left( {1 + {s/\omega_{i\; n}}} \right)\left( {1 + {s/\omega_{out}}} \right)}} & (3)\end{matrix}$where R_(D) is the drain (series) resistance, ω_(in) is the angularfrequency at the input pole, ω_(out) is the angular frequency at theoutput pole, and s is the complex frequency operator equal to jω forwhich ω is the angular frequency. The parasitic capacitances of IGFET Qin the common-source configuration enter Eq. 3 by way of polefrequencies ω_(in) and ω_(out) given respectively as:

$\begin{matrix}{\omega_{i\; n} = \frac{1}{R_{S}\left\lbrack {C_{GS} + {\left( {1 + {g_{m}R_{D}}} \right)C_{GD}}} \right\rbrack}} & (4) \\{\omega_{out} = \frac{1}{R_{D}\left( {C_{DB} + C_{GD} + C_{L}} \right)}} & (5)\end{matrix}$

Parasitic drain-to-body capacitance C_(DB) appears in output polefrequency ω_(out) of Eq. 5. For the situation in which source resistanceR_(S) is zero in the common-source configuration, input pole frequencyω_(in) is infinite in accordance with Eq. 4. The bandwidth of IGFET Q inFIG. 51 a then equals ω_(out) as given by Eq. 5. Output pole frequencyω_(out) increases with increasing drain-to-body capacitance C_(DB) forgiven values of drain resistance R_(D) and parasitic gate-to-draincapacitance C_(GD). Reducing parasitic drain-to-body capacitance C_(DB)thus desirably increases the bandwidth for the common-sourceconfiguration of IGFET Q in FIG. 51 a.

Also, parasitic drain-to-body capacitance C_(DB) in the common-sourceconfiguration is in parallel with load capacitance C_(L) as shown inFIG. 51 a. Reducing drain-to-body capacitance C_(DB) thus advantageouslyreduces its output loading effect.

Transfer function H_(A) for the common-gate amplifier configuration ofFIG. 51 b is the input-pole/output-pole function:

$\begin{matrix}{H_{A} = \frac{\left( {g_{m} + g_{mb}} \right)}{\left\lbrack {1 + {\left( {g_{m} + g_{mb}} \right)R_{S}}} \right\rbrack\left( {1 + {s/\omega_{i\; n}}} \right)\left( {1 + {s/\omega_{out}}} \right)}} & (6)\end{matrix}$where input pole frequency ω_(in) for the common-gate configuration isgiven as:

$\begin{matrix}{\omega_{i\; n} = \frac{1 + {\left( {g_{m} + g_{mb}} \right)R_{S}}}{R_{S}\left( {C_{GS} + C_{SB}} \right)}} & (7)\end{matrix}$Reducing source-to-body capacitance C_(SB) causes input pole frequencyω_(in) to increase. This enables the performance of IGFET Q to improvein the common-gate configuration of FIG. 51 b.

Output pole frequency ω_(out) is given by Eq. 5 for the common-gateamplifier configuration of FIG. 51 b. Reducing parasitic drain-to-bodycapacitance C_(DB) therefore increases the bandwidth of the common-gateconfiguration of FIG. 51 b.

Transfer function H_(A) for the common-drain amplifier configuration ofFIG. 51 c is the single-zero/single-pole function:

$\begin{matrix}{H_{A} = \frac{\left( {1 + {s/\omega_{z}}} \right)}{\left( {1 + {s/\omega_{p}}} \right)}} & (8)\end{matrix}$where ω_(z) is the angular frequency at the zero, and ω_(p) is theangular frequency at the pole. The parasitic capacitances enter Eq. 8 byway of zero frequency ω_(z) and pole frequency ω_(p) given respectivelyas:

$\begin{matrix}{\omega_{z} = \frac{g_{m}}{C_{GS}}} & (9) \\{\omega_{p} = \frac{g_{m}}{{g_{m}R_{S}C_{GD}} + C_{SB} + C_{GS} + C_{L}}} & (10)\end{matrix}$Parasitic source-to-body capacitance C_(SB) appears in pole frequencyω_(p) of Eq. 10. By reducing capacitance C_(SB), pole frequency ω_(p)increases. This improves the frequency characteristics of IGFET Q in thecommon-drain configuration of FIG. 51 c.

Similar to what occurs with drain-to-body capacitance C_(DB) in thecommon-source configuration of FIG. 51 a, parasitic source-to-bodycapacitance C_(SB) is in parallel with load capacitance C_(L) in thecommon-drain configuration as shown in FIG. 51 c. Reducingsource-to-body capacitance C_(SB) thus beneficially reduces its loadingeffect in the common-drain configuration.

FIG. 52 illustrates a small-signal model of the shorted-output versionof the common-source amplifier configuration of FIG. 51 a. In thesmall-signal model of FIG. 52, drain electrode D of IGFET Q iselectrically shorted to source electrode S. FIG. 53 presents asmall-signal equivalent circuit of the model of IGFET Q of FIG. 52.Element 448 in FIG. 53 is a voltage-controlled current source. Itemsv_(gs), i_(i), and i_(o), in FIGS. 52 and 53 respectively are thesmall-signal gate-to-source (input) voltage, the small-signal inputcurrent, and the small-signal output current.

Cut-off frequency f_(T) of an IGFET is defined as the value of frequencyfat which the absolute value of current gain A_(I) of the IGFET'sshorted-output common-source configuration falls to 1. That is,

$\begin{matrix}{{{{{A_{I}(f)}} = \frac{{I_{out}(f)}}{I_{i\; n}}}}_{V_{out} = 0} = 1} & (11)\end{matrix}$Cut-off frequency f_(T) is derived from the small-signal equivalentcircuit of FIG. 53 as:

$\begin{matrix}{f_{T} = \frac{g_{m}}{2\;{\pi\left( {C_{GS} + C_{GD} + C_{GB}} \right)}}} & (12)\end{matrix}$Capacitance C_(GB) in Eq. 12 is the parasitic capacitance between gateelectrode G and the IGFET's body region outside the active area occupiedby IGFET Q.

Increasing transconductance g_(m) of an amplifying IGFET generallyimproves its performance capability because its voltage gain generallyincreases. Since cut-off frequency f_(T) increases with increasingtransconductance g_(m) according to Eq. 12, an increase in cut-offfrequency f_(T) is an indicator of improved IGFET performance.

In the classical long-channel model of IGFET Q for which sourceresistance R_(S) is zero, transconductance g_(m) is:

$\begin{matrix}{g_{m} = {\left( \frac{W}{L} \right)\mu_{n}{C_{GIa}\left( {V_{GS} - V_{T}} \right)}}} & (13)\end{matrix}$where W is the IGFET width, L again is the channel length, μ_(n) is theelectron mobility, and C_(GIa) is the gate dielectric capacitance perunit area. In the short-channel velocity-saturation model of IGFET Q,transconductance g_(m) is:g_(m)=Wv_(nast)C_(GIa)  (14)where v_(nsat) is the electron saturation velocity since IGFET Q is ann-channel device. Examination of Eqs. 13 and 14 shows thattransconductance g_(m) is proportional to areal gate dielectriccapacitance C_(GIa) in both the long-channel and short-channel models.

For the classical long-channel model of IGFET Q in saturation,capacitances C_(GS), C_(GD), and C_(GB) are:

$\begin{matrix}{C_{GS} = {{\left( \frac{2}{3} \right){WLC}_{GIa}} + {{WL}_{GSoverlap}C_{GIa}}}} & (15) \\{C_{GD} = {{WL}_{GDoverlap}C_{GIa}}} & (16) \\{C_{GB} = {WLC}_{GIa}} & (17)\end{matrix}$where L_(GSoverlap) and L_(GDoverlap) are the longitudinal distancesthat the gate electrode respectively overlaps the source and drain ofIGFET Q. The term WL_(GSoverlap)C_(GIa) is the parasitic capacitancearising from the gate electrode overlapping the source. The termWL_(GDoverlap)C_(GIa) is the parasitic capacitance arising from the gateelectrode overlapping the drain. Inserting Eqs. 15-17 into Eq. 12 yieldscut-off frequency f_(T) for an ideal long-channel IGFET in saturation.

Eqs. 15 and 16 are not expected to be accurate for the asymmetric IGFETsof the invention due to the asymmetric longitudinal dopant grading intheir channel zones. However, Eqs. 15 and 16 can be used as trendindicators in calculating parasitic capacitances C_(GS) and C_(GD) forassessing cut-off frequencies f_(T) of the present asymmetric IGFETs.More accurate values of capacitances C_(GS) and C_(GD) can be determinedby computer simulation.

Cut-off frequency f_(T) involves, by definition, the short-circuitcondition at the output in the common-source configuration. As a result,frequency f_(T) essentially obliterates the effects of parasiticdrain-to-body capacitance C_(DB). Also, frequency f_(T) does not reflectthe effect of parasitic source-to-body capacitance C_(SB) since itutilizes the common-source configuration.

Cut-off frequency f_(T) and has a peak cut-off value f_(Tpeak) dependenton the operating current, i.e., drain current I_(D). Although peakcut-off frequency f_(Tpeak) is useful in assessing high-frequency IGFETperformance, circuits typically operate at frequencies one to twofactors of 10 lower than peak value f_(Tpeak). In addition to peak valuef_(Tpeak) desirably being high for an IGFET, it is generally desirablethat there be reduced variation of cut-off frequency f_(T) withdecreasing operating current below operating current levelscorresponding to peak value f_(Tpeak).

Source-body and drain-body junctions, such as pn junctions 110 and 112in inventive IGFETs 100, 140, 150, 160, 170, 180, 190, 210, 100V, 140V,150V, 160V, 170V, 180V, and 190V, are normally reverse biased. When a pnjunction is in reverse bias, the depletion region along the junctionexhibits a small-signal areal capacitance C_(da) given as:

$\begin{matrix}{C_{da} = \frac{K_{S}ɛ_{0}}{t_{d}}} & (18)\end{matrix}$where ∈₀ is the absolute permittivity, K_(S) is the relativepermittivity of the semiconductor material, and t_(d) is thevoltage-dependent thickness of the depletion region.

For a pn junction formed along a uniformly doped substrate, thicknesst_(d) for the depletion region of such an ideal pn junction is:

$\begin{matrix}{t_{d} = \sqrt{\frac{2\; K_{S}{ɛ_{0}\left( {V_{R} + V_{BI}} \right)}}{{qN}_{B}}}} & (19)\end{matrix}$where V_(R) is the applied reverse voltage, V_(BI) is the built-involtage of the junction, q is the electronic charge, and N_(B0) is theuniform background dopant concentration in the substrate. Built-involtage V_(BI) varies with background dopant concentration N_(B0)according to the relationship:

$\begin{matrix}{V_{BI} = {\left( \frac{2\;{kT}}{q} \right){\ln\left( \frac{N_{B\; 0}}{n_{i}} \right)}}} & (20)\end{matrix}$where k is Boltzmann's constant, T is the temperature, and n_(i) is theintrinsic carrier concentration.

FIG. 54 illustrates how net dopant concentration N_(D)-N_(A) varies withdistance y into p-type substrate material of a model of a pn junctionthat can have any of three basic types of dopant profiles in the p-typesubstrate, where N_(D) and N_(A) respectively are the absolute donor andacceptor dopant concentrations. The junction model is also shown in FIG.54. As indicated by the illustrated junction model, the p-type materialis thicker, and thus more lightly doped, than the junction's n-typematerial. Curves 450, 452, and 454 in FIG. 54 respectively indicateexamples of hypoabrupt, flat, and hyperabrupt dopant profiles in thep-type material. Distance y_(d) indicates the thickness of the p-typeportion of the depletion region along the junction.

Hypoabrupt profile curve 450 approximately represents the verticaldopant profile below drain-body junction 112 in each of inventiven-channel IGFETs 100, 140, 150, 160, 170, 180, 190, 210, 100V, 140V,150V, 160V, 170V, 180V, and 190V. In IGFETs 150, 160, 180, 190, 210,150V, 160V, 180V, and 190V where drain 104 includes main drain portion104M and lateral drain extension 104E, curve 450 specifically representsthe vertical dopant profile below the portion of drain-body junction 112along the bottom of main portion 104M. For IGFETs 170, 180, 190, 170V,180V, and 190V in which source 102 extends deeper below the uppersemiconductor surface than pocket portion 120, curve 450 also representsthe vertical dopant profile below source-body junction 110, specificallybelow the junction portion along the bottom of main source portion 104Mfor each of IGFETs 180, 190, 180V, and 190V. Subject to the conductivitytypes being reversed, curve 450 further represents the vertical dopantprofile below the bottom of main portion 264M of drain 264 in each ofinventive p-channel IGFETs 220, 220U, and 220V. Flat curve 452represents the p-type material of the ideal pn junction covered by Eqs.18-20.

FIG. 55 depicts how parasitic areal capacitance C_(da) of the depletionregion varies with reverse voltage V_(R) across the pn junction modeledin FIG. 54. Curves 460, 462, and 464 in FIG. 55 indicate the C_(da)variations respectively for curves 450, 452, and 454 in FIG. 54. Inparticular, curve 462 qualitatively indicates the power-law variation ofcapacitance C_(da) for the ideal pn junction as determined from Eq. 18using the t_(d) data from Eq. 19 (and the V_(BI) data from Eq. 20).

Parasitic drain-to-body capacitance C_(DB) along drain-body junction 112in inventive n-channel IGFET 100, 140, 150, 160, 170, 180, 190, 210,100V, 140V, 150V, 160V, 170V, 180V, or 190V is approximatelyproportional to areal depletion capacitance C_(da) as represented inFIG. 55 by curve 460 corresponding to hypoabrupt-profile curve 450 inFIG. 54. As FIG. 55 shows, depletion capacitance C_(da) is lower forcurve 460 than (a) for curve 462 corresponding to flat-profile curve 452in FIG. 54 or (b) for curve 464 corresponding to hyperabrupt-profilecurve 454 in FIG. 54. Hence, the hypoabrupt vertical dopant profilebelow drain-body junction 112 in each IGFET 110, 140, 150, 160, 170,180, 190, 210, 100V, 140V, 150V, 160V, 170V, 180V, or 190V causes itsdrain-to-body capacitance C_(DB) to be reduced. The same applies tocapacitance C_(DB) along the bottom of drain 264 in inventive p-channelIGFET 220, 220U, or 220V. Parasitic source-to-body capacitance C_(SB) isalso normally reduced, especially in n-channel IGFETs 170, 180, 190,170V, 180V, and 190V where source 102 extends deeper below the uppersemiconductor surface than pocket portion 120.

Additionally, areal depletion capacitance C_(da) for curve 460corresponding to hypoabrupt-profile curve 450 in FIG. 54 varies moreslowly with reverse voltage V_(R) than curve 462 or 464 as indicated bycomparing curve 460 in FIG. 55 to curves 462 and 464. Parasiticdrain-to-body capacitance C_(DB) in each inventive IGFET 100, 140, 150,160, 170, 180, 190, 210, 100V, 140V, 150V, 160V, 170V, 180V, 190V, 220,220U, or 220V thus has reduced variance with reverse voltage V_(R). Thisis advantageous because less compensation is needed to account forvariations in capacitance C_(DB). The same comments apply to parasiticsource-to-body capacitance C_(SB), particularly in IGFETs 170, 180, 190,170V, 180V, and 190V.

In further examining the hypoabrupt dopant profile below drains 104 and204, consider the extreme example of a hypoabrupt junction profile inwhich net dopant concentration N_(B) of the semiconductor material alongthe more lightly doped side of the pn junction makes a step change froma first dopant-concentration value to a higher seconddopant-concentration value at a selected distance sufficiently close tothe junction to affect the parasitic capacitance along the junction.This example is modeled in FIG. 56 which illustrates how net dopantconcentration N_(B) varies with distance y from the junction.

The two-step pn junction model of FIG. 56 is configured in the followingway. The more lightly doped side of the pn junction is formed withp-type material in which net dopant concentration N_(B) is at firstvalue N_(B0) for a distance extending from the junction out to adistance y_(d0) at which concentration N_(B) makes a step increase to asecond value N_(B1). Distance y_(d0) is the location that wouldconstitute the p-type boundary of the depletion region at a zero valueof reverse voltage V_(R) if concentration N_(B) in the p-type materialwere at low value N_(B0) beyond distance y_(d0) out at least to alocation beyond which changes in concentration N_(B) of the p-typematerial would not significantly affect the parasitic capacitance alongthe junction.

The depletion region along the junction in the model of FIG. 56 expandsfrom distance y_(d0) to a maximum distance y_(dmax) as reverse voltageV_(R) increases from zero to some maximum value V_(Rmax) Beyond distancey_(dmax), net dopant concentration N_(B) can have an arbitrary profilein the p-type material as indicated in FIG. 56. As also indicated inFIG. 56, concentration N_(B) of the more heavily doped n-type materialalong the junction is at a uniform value N_(D0) much greater thanN_(B1).

FIG. 56 depicts the dopant profile in the p-type material as highconcentration value N_(B1) ranges from N_(B0) (at which the step changein concentration at distance y_(d0) disappears) up to 20 N_(B0) at atypical y_(d0) value of 0.2 μm and a typical N_(B0) value of 3×10¹⁶atoms/cm³. If the modeled pn junction is the drain-body junction atdepth y_(D) below the upper semiconductor surface, the step increase inconcentration N_(B) from N_(B0) to N_(B1) occurs at a depth y_(D)+y_(d0)below the upper surface.

Areal depletion capacitance C_(da) of the two-step pn junction of FIG.56 is governed by the differential equation:

$\begin{matrix}{\frac{\mathbb{d}V_{R}}{\mathbb{d}\left( {1/C_{da}^{2}} \right)} = \frac{{qK}_{S}ɛ_{0}N_{B}{t_{d}\left( V_{R} \right)}}{2}} & (21)\end{matrix}$Integrating Eq. 21 subject to the condition that the depletion regionextend to distance y_(d0) when reverse voltage V_(R) is zero yields thefollowing value for distance y_(d0):

$\begin{matrix}{y_{d\; 0} = \sqrt{\frac{2\; K_{S}ɛ_{0}V_{BI}}{{qN}_{B\; 0}}}} & (22)\end{matrix}$Combining Eqs. 18 and 22 produces the following result for depletioncapacitance C_(da):

$\begin{matrix}{C_{da} = {\sqrt{\frac{{qK}_{S}ɛ_{0}N_{B\; 0}}{2\left( {V_{BI} + {V_{R}{N_{B\; 0}/N_{B\; 1}}}} \right)}}.}} & (23)\end{matrix}$

FIG. 57 illustrates how areal depletion capacitance C_(da) varies withreverse voltage V_(R) for values of high concentration value N_(B1)ranging from N_(B0) (again, beyond the modeled junction) up to 20 N_(B0)as determined from Eq. 23. FIG. 57 shows that increasing concentrationratio N_(B1)/N_(B0) causes capacitance C_(da) to vary more slowly withvoltage V_(R). For this reason, it is desirable that concentration ratioN_(B1)/N_(B0) be as high as reasonably possible in order to haveparasitic capacitances C_(DB) and C_(SD) vary slowly in the asymmetricIGFETs of the invention.

Areal depletion capacitance is at an initial value C_(d0a) when reversevoltage V_(R) is zero. Setting voltage V_(R) to zero in Eq. 23 yields:

$\begin{matrix}{C_{{da}\; 0} = \sqrt{\frac{{qK}_{S}ɛ_{0}N_{B\; 0}}{2\; V_{BI}}}} & (24)\end{matrix}$

Initial depletion capacitance value C_(d0a) is, as expected, theclassical value for an ideal pn junction at zero reverse voltage. Inaccordance with Eq. 24, capacitance value C_(d0a) decreases withdecreasing low concentration value N_(B0) according to the square rootof low value N_(B0). In combination with choosing a high value ofconcentration ratio N_(B1)/N_(B0) so as to have slow variations inparasitic capacitances C_(DB) and C_(SB), low concentration value N_(B0)should be low in order for capacitances C_(DB) and C_(SB) to be low atzero reverse voltage V_(R).

Computer Simulations Relating to Capacitance and Frequency Parameters

With the foregoing information about capacitance and frequencyparameters in mind, small-signal simulations were performed with theMedici simulator to characterize the junction capacitances of inventivestructure A. FIGS. 58 a and 58 b respectively depict short-channel andlong-channel versions of structure A created by the Medici simulator forthe junction capacitance characterization. Items 470 and 472respectively indicate the source and drain contacts (or source and drainelectrodes) in FIGS. 58 a and 58 b. Metal silicide layers 254 and 256are respectively subsumed in contacts 470 and 472. The doping contour ofp-type body material (or region) 108 in each of FIGS. 58 a and 58 billustrates the graded nature of the doping in body material 108. Theshort-channel IGFET of FIG. 58 a had a gate length L_(G) of 0.15 μm.Gate length L_(G) for the long-channel IGFET of FIG. 58 b was 1.0 μm.

FIG. 59 illustrates parasitic lineal drain-to-body capacitance C_(DBw)as a function of drain-to-body voltage V_(SB) for a short-channelimplementation of inventive structure A quite similar to theshort-channel implementation of structure A in FIG. 58 a and for ashort-channel implementation of reference structure B substantiallycorresponding size-wise and, except for the doping features of theinvention, dopant-wise to the short-channel implementation of structureA in FIG. 59. Gate length L_(G) is 0.2 μm in FIG. 59 rather than 0.15 μmas in FIG. 58 a. Gate-to-source voltage V_(GS) was 0.9 V for the C_(DBw)simulations of FIG. 59. As FIG. 59 shows, drain-to-body capacitanceC_(DBw) is considerably lower for this short-channel version ofinventive structure A than for the corresponding short-channel versionof reference structure B. In particular, capacitance C_(DBw) for theexamined short-channel version of inventive structure A wasapproximately 50% of capacitance C_(DBw) for the examined short-channelversion of reference structure B in the V_(DS) range of 0 to 2 V.

FIG. 60 depicts parasitic lineal source-to-body capacitance C_(SBw) as afunction of source-to-body voltage V_(SB) for the short-channelimplementations of structures A and B examined in FIG. 59.Gate-to-source voltage V_(GS) was again 0.9 V. As shown in FIG. 60,source-to-body capacitance C_(SBw) was considerably lower for theexamined short-channel version of inventive structure A than for thecorresponding short-channel version of reference structure B. Althoughthe C_(SBw) reduction was not as great as the C_(DBw) reduction, theexamined short-channel version of structure A had approximately a 35-40%lower C_(DBw) value than the examined short-channel version of structureB at the V_(SB) value of 2.0 V, and a 25-35% lower C_(DBw) value thanthe examined short-channel version of reference structure B at theV_(SB) value of 0 V.

The somewhat lesser improvement in drain-to-body capacitance C_(DBw) forthe short-channel version of structure A examined in FIG. 59 is expectedbecause the total p-type dopant in source 102 is increased by the p-typepocket implant. Also, source-to-body capacitance C_(SB) is lessimportant than drain-to-body capacitance C_(DB) in many applicationsbecause source 102 is shorted to body material 108. As desired, furtherreduction in source-to-body capacitance C_(SB) for short-channelversions of structure A can be achieved by making well portion 116deeper.

FIG. 61 illustrates cut-off frequency f_(T) as a function of linealdrain current I_(Dw) for the short-channel implementations of structuresA and B examined in FIG. 59. FIG. 61 also illustrates the variation ofcut-off frequency f_(T) with lineal drain current I_(Dw) for a variationA′ of inventive short-channel structure A. In FIG. 61 and in latergraphs that present computer-simulated data for inventive structures Aand A′, curves representing data for structure A′ are marked with solidcircles to distinguish that data from the data marked with empty circlesfor structure A. The particular characteristics of further inventivestructure A′ are described below in connection with FIGS. 63 and 64. AsFIG. 61 indicates, cut-off frequency f_(T) was largely the same forsimulated short-channel structures A, A′, and B.

The variation of cut-off frequency f_(T) with lineal drain currentI_(Dw) is illustrated in FIG. 62 for long-channel versions of theimplementations of structures A, A′, and B in FIG. 61. As FIG. 62 shows,cut-off frequency f_(T) was largely the same for the long-channelversions of inventive structures A and A′. Importantly, frequency f_(T)for the long-channel versions of inventive structures A and A′ wasconsiderably greater than for the long-channel version of referencestructure B. Accordingly, the long-channel versions of inventivestructures A and A′ had better performance capability than thelong-channel version of reference structure B.

Additional IGFET in which Vertical Body-Material Dopant Profile BelowDrain is Hypoabrupt Due to Subsurface Maximum in Well DopantConcentration

FIG. 63 illustrates a short n-channel implementation 480 of asymmetricIGFET structure A′ according to the invention. In addition to generallyshowing structural details, doping contours for IGFET 480 are depictedin FIG. 63 as a function of depth y and longitudinal distance x from asource location. The source location for measuring distance x isapproximately 0.35 μm from the channel-zone center.

IGFET 480 is configured generally similar to short-channel IGFET 140 ofFIG. 11 except that source 102 in IGFET 480 consists of n++ main sourceportion 102M and n+ more lightly doped lateral extension 102E as inlong-channel IGFET 150 of FIG. 13. This enables source resistance R_(S)to be reduced in IGFET 480 and thereby improves its analog performance.As in IGFETs 140 and 150, p-type pocket portion 120 extends deeper belowthe upper semiconductor surface than source 102. Drain depth y_(D) forIGFET 480 is approximately 50% greater than source depth Y_(s).

FIG. 64 presents net dopant concentration N_(N) along the uppersemiconductor surface as a function of distance from the precedingsource location for inventive structure A of FIG. 39 and inventivestructure A′, i.e., IGFET 480, of FIG. 63. As in FIGS. 12 c and 14 c,curve segments 106* and 120* here represent concentration N_(N) of thenet p-type dopant in respective regions 106 and 120 while curve segments102M*, 102E*, 104M*, 104E*, and 104* represent concentrations N_(N) ofthe net n-type dopant in respective regions 102M, 102E, 104M, 104E, and104. Although only marked with open circles, curve segments 102M*,102E*, 106*, and 120* apply to both structure A and structure A′.

As curve segments 104* and 102M* in FIG. 64 indicate, short-channelIGFET 480 of inventive structure A′ reaches a somewhat lower maximum netdopant concentration in drain 104 along the upper surface than in mainsource portion 102M. More particularly, the maximum value of net dopantconcentration N_(N) in drain 104 along the upper surface of IGFET 480 isnormally 20-50%, typically 30-40%, of the maximum upper-surface value ofconcentration N_(N) in main source portion 102M. Although FIG. 64illustrates an example in which the maximum upper-surface value ofconcentration N_(N) in drain 104 slightly exceeds 1×10²⁰ atoms/cm², themaximum upper-surface N_(N) concentration in drain 104 of short-channelIGFET 480 can readily be considerably lower, e.g., 5×10¹⁹ atoms/cm² downto 1×10¹⁹ atoms/cm² or less, dependent on the maximum upper-surfaceN_(N) concentration in main source portion 102M. Also, drain 104 inIGFET 480 extends somewhat deeper below the upper surface than mainsource portion 102M. In essence, the two-part drain formed with a mainportion and a more lightly doped lateral extension in an IGFET, such asIGFET 150, whose source consists of a main portion and a more lightlydoped lateral extension is replaced in IGFET 480 with a deeper morelightly doped drain. The reduced doping in drain 104 of IGFET 480results in a lower electric field in drain 104 and enables IGFET 480 tooperate away from electric-field magnitudes at which undesirable drainimpact ionization occurs.

FIG. 65 illustrates threshold voltage V_(T) as a function of gate lengthL_(G) for computer simulations of the IGFETs of inventive structure A′,reference structure B, and a further symmetric reference structure Dlacking both halo pocket portions of structure B but otherwisesubstantially identical size-wise and dopant-wise to structure B. Gatedielectric thickness t_(GI) was 4.0 nm in the simulations of FIG. 65.

As FIG. 65 shows, threshold voltage roll-off was shifted to a lowervalue of threshold voltage V_(T) in inventive structure A′ than inreference structure B or D. FIG. 65 also shows that inventive structureA′ incurred less undesired reverse short-channel effect than referencestructure B or D. That is, inventive structure A underwent less change,normally less decrease, in threshold voltage V_(T) with increasing gatelength L_(G) in the long-channel domain than reference structure B or D.Structure A thus had better short-channel and long-channelcharacteristics than structure B or D.

Fabrication of Additional IGFET

N-channel IGFET 480 which implements asymmetric IGFET structure A′ ofFIG. 63 is typically fabricated in accordance with the inventionaccording to the steps used to fabricate asymmetric n-channel IGFET 210in the process of FIG. 31 subject to suitable modifications in then-type source/drain extension and main source/drain implant steps andsubject to the use of an additional masking step and associated ionimplant operation. These differences are described below for IGFET 480using, as appropriate, the same reference notation employed indescribing the fabrication of IGFET 210.

In particular, N+ precursor source extension 102EP is defined for IGFET480 at the stage of FIG. 311 without defining a corresponding n+precursor drain extension for IGFET 480. This entails configuringphotoresist mask 422 to extend above the location where a precursordrain extension would otherwise be formed for IGFET 480 but to have anopening above the location for precursor source extension 102EP forIGFET 480. In so doing, photoresist 422 is critically aligned toprecursor gate electrode 128P for IGFET 480. The n-type source/drainextension implant is performed as described above in connection withFIG. 311 after which photoresist 422 is removed. Because photoresist 422masked the location for a precursor drain extension for IGFET 480,precursor source extension 102EP is formed for IGFET 480 without forminga corresponding precursor drain extension.

Later at the stage of FIG. 31 q, photoresist mask 434 is configured toextend above the location for drain 104 of IGFET 480 but to have anopening above the location for main source zone 102M of IGFET 480.Photoresist 434 is critically aligned to precursor gate electrode 128Pof IGFET 480. The n-type main source/drain implant is performed asgenerally described above in connection with FIG. 31 q after whichphotoresist 434 is removed. Since photoresist 422 masked the locationfor drain 104 of IGFET 480, main source zone 102M is defined for IGFET480 without yet defining drain 104. The portion of precursor sourceextension 102EP outside mains source zone 102M constitutes sourceextension 102E. With part of precursor gate electrode 128P of IGFET 480uncovered during the implant, the n-type main source/drain dopant alsoentered the uncovered part of that electrode 128P.

An additional photoresist mask (not shown) having an opening above theintended location for source 102 of IGFET 480 is formed on dielectriclayers 430 and 432 and gate sidewall spacer 252 for IGFET 480, gatesidewall spacers 250 and 252 for IGFET 210, and gate sidewall spacers290, 292, 330, 332, 370, and 372. The additional photoresist iscritically aligned to precursor gate electrode 128P of IGFET 480. N-typedrain dopant is ion implanted at a very high dosage through theuncovered portion of surface dielectric layer and into the underlyingmonosilicon to define n++ drain 104 of IGFET 480. Although the dosage ofthe n-type drain dopant used to define drain 104 of IGFET 480 is veryhigh, the dosage of the n-type drain dopant is less than the very highdosage of the n-type main source/drain dopant used to define main sourcezone 102M of IGFET 480. Consequently, drain 104 of IGFET 480 is morelightly doped than its main source zone 102M.

The n-type drain implant for IGFET 480 is also performed under suchconditions that its drain 104 extends deeper below the uppersemiconductor surface than both its main source zone 102M and itsprecursor source extension 102EP. For instance, the n-type mainsource/drain implant and the n-type drain implant for IGFET 480 can beperformed with the same n-type dopant, either arsenic or antimony. Inthis case, the n-type drain implant for IGFET 480 is performed at ahigher implant energy than the n-type main source/drain implant.Alternatively, the two implants can be performed using different n-typedopants with the n-type drain dopant of IGFET 480 being of lowermolecular weight than the n-type main source/drain dopant. In oneexample, arsenic is the main source/drain dopant while phosphorus isn-type drain dopant of IGFET 480. The implant energies are closer toeach other in this alternative than in the first-mentioned case.However, the range of the n-type drain dopant is greater than the rangeof the n-type main source/drain dopant in both cases. The additionalphotoresist is removed after the n-type drain implant for IGFET 480.

The part of precursor gate electrode 128P for IGFET 480 covered duringthe main source/drain implant was largely uncovered during the n-typedrain implant for IGFET 480. This enabled the n-type drain dopant forIGFET 480 to enter the part of that electrode 128P covered during then-type main source/drain implant. As a result, substantially all ofprecursor gate electrode 128P of IGFET 480 is now heavily doped n-type.Precursor gate electrode 128P of IGFET 480 thereby becomes its n++ gateelectrode 128.

The n-type drain implant for IGFET 480 can be performed before, ratherthan after, the n-type main source/drain implant. In either case, theremainder of the fabrication of IGFET 480 is performed as describedabove for IGFET 210.

If IGFET 210 is also to be present in the semiconductor structure, theconfigurations of photoresist masks 422 and 434 above the intendedlocation for IGFET 210 are the same as described above respectively inconnection with FIGS. 31 l and 31 q. The formation of IGFET 480 does notaffect the formation of IGFET 210.

Further Complementary-IGFET Structure Suitable for Mixed-SignalApplications

FIG. 66 illustrates a variation, in accordance with the invention, ofthe complementary-IGFET structure of FIG. 29.1. The complementary-IGFETstructure of FIG. 66 is especially suitable for mixed-signalapplications. The principal configurational difference between thecomplementary-IGFET structures of FIGS. 29.1 and 66 is that thecomplementary-IGFET structure of FIG. 66 is created from a startingstructure such as a bonded wafer.

In the complementary-IGFET structure of FIG. 66, a subsurfaceelectrically insulating layer 482 typically consisting primarily ofsilicon oxide separates a lower semiconductor layer 484 from an uppersemiconductor layer having islands 202 and 204 laterally separated byfield-insulation region 200 along the upper semiconductor surface. Lowersemiconductor layer 484 normally consists of monosilicon, either p typeor n type. FIG. 66 presents an example in which lower semiconductorlayer 484 is lightly doped p type. An electrically insulating extension486, typically of the trench type and likewise typically consistingprimarily of silicon oxide, extends from field-insulation region 200 tosubsurface insulating layer 482. Field insulation 200 and insulatingextension 486 together laterally surround islands 202 and 204 so thatthey are fully dielectrically isolated from each other.

Islands 202 and 204 normally consist of doped <100> monosilicon. Island202 has a low substantially uniform n-type background dopantconcentration on which is imposed a low, but slightly higher,substantially uniform p-type background concentration typically providedby the p-type semiconductor dopant aluminum. Consequently, portions ofisland 202 that do not receive any other dopant (p type or n type) arelightly doped p type. Island 204 simply has a low substantially uniformn-type background dopant concentration.

Island 202 provides the monosilicon for a variation 210W of longn-channel IGFET 210. Source 102 and drain 104 of long n-channel IGFET210W are separated by a channel portion of p-type body material 108consisting of a lightly doped lower portion 488, p+ well portion 116,and an upper portion 490. P− lower body-material portion 488 and p-typeupper body-material portion 490 respectively correspond to p− lowerbody-material portion 114 and p-type upper body-material portion 118 ofIGFET 210. Upper body-material portion 490 of IGFET 210W consists ofsource-contacting p+ pocket portion 120 and lightly doped remainder 492that corresponds to p− upper body-material remainder 124 of IGFET 210.Due to the imposition of the low p-type background dopant concentrationon the lower n-type background concentration in island 202 of IGFET210W, net dopant concentration N_(N) in the bulk of each region 488 or492 is largely the difference between the p-type and n-type backgrounddopant concentrations.

Aside from the above-mentioned configurational differences and thepresence of the two background dopant concentrations in island 202,n-channel IGFET 210W is configured and constituted substantially thesame as n-channel IGFET 210. P− lower body-material portion 488 can bedeleted so that p+ well portion 116 extends down to subsurfaceinsulating layer 482.

Island 204 provides the monosilicon for a variation 220W of longp-channel IGFET 220. Source 262 and drain 264 of long p-channel IGFET220W are separated by a channel portion of n-type body material 268consisting of a lightly doped lower portion 494, n+ well portion 276,and an upper portion 496 corresponding to n-type upper body-materialportion 278 of IGFET 220. Upper body-material portion 496 of IGFET 220Wconsists of source-contacting n+ pocket portion 280 and lightly dopedremainder 498 that corresponds to n− upper body-material remainder 284of IGFET 220. Unlike IGFET 220, IGFET 220W does not have a low p-typebackground dopant concentration and does not utilize an n-typecompensating dopant to ensure that all of upper body-material portion496 is of n-type conductivity. Net dopant concentration N_(N) in thebulk of each region 494 or 498 is simply the n-type background dopantconcentration.

Aside from the above-mentioned configurational differences and theabsence of an n-type compensating dopant to ensure that all of upperbody-material portion 496 is n type, p-channel IGFET 220W is configuredand constituted substantially the same as p-channel IGFET 220. N-lowerbody-material portion 494 can be deleted so that n+ well portion 276extends down to subsurface insulating layer 482.

Fabrication of Further Complementary-IGFET Structure

The complementary-IGFET structure of FIG. 66 is manufactured in thefollowing manner according to the invention. A structure is firstprovided in which subsurface insulating layer 482 is sandwiched betweenlower semiconductor layer 484 and an upper semiconductor regionconsisting of <100> n-type monosilicon at a low uniform dopantconcentration. This initial structure can be created, for example, bybonding two semiconductor wafers together through electricallyinsulating material that forms subsurface insulating layer 482. One ofthe wafers provides the <100> n-type monosilicon for the uppersemiconductor region. The other wafer provides lower semiconductor layer484 again normally consisting of monosilicon, either p type as in theillustrated example or n type.

Insulating extension 486 is formed in the n− upper semiconductor regionaccording to a deep trench-isolation technique. Field-insulating region200 is then formed along the outside (upper) surface of the n− uppersemiconductor region according to a shallow trench-isolation techniqueto define islands 202 and 204. Using a photoresist mask having anopening above island 202, p-type semiconductor dopant normallyconsisting of aluminum is introduced into island 202 at a light dosagethat is sufficiently high to convert all the material of island 202 top-type conductivity at a low net concentration. When aluminum is used toperform the p-type doping of island 202, the aluminum diffusesrelatively fast throughout island 202 so that it becomes substantiallyuniformly doped p type in a relatively short time.

P+ well portion 116 and n+ well portion 276 are formed respectively inislands 202 and 204 in the manner described above in connection withfabrication of IGFETs 210 and 220. Part of island 202 underlies well 116and constitutes p-lower body-material portion 488. Part of island 204similarly underlies well 276 and constitutes n-lower body-materialportion 494. Regions 102, 104, 120, 126, 128, 250, 252, 254, 256, and258 for IGFET 210W and regions 262, 264, 280, 286, 288, 290, 292, 294,296, and 298 for IGFET 220W are then formed as described above forIGFETs 210 and 220. The p-type monosilicon above well portion 116constitutes p-type upper body-material portion 490 of which the partoutside p+ halo pocket portion 120 constitutes p− upper body-materialremainder 492. The n-type monosilicon above well portion 276 constitutesp-type upper body-material portion 496 of which the part outside n+ halopocket portion 280 constitutes n− upper body-material remainder 498.

IGFETs in which Vertical Body-Material Dopant Profile Below Drain isHypoabrupt Due to Step Change in Body-Material Dopant Concentration

The vertical dopant profiles below the drains in the asymmetric IGFETsconfigured according to the invention can be made hyperabrupt in waysother than having concentration N_(T) of the conductivity-type-definingdopant in the body material decrease by at least a factor of 10 in goingfrom the location of the maximum well dopant concentration up to thedrain. In particular, the vertical dopant profile below the drain can bemade hypoabrupt by arranging for the body material below the drain toinclude (a) a drain-adjoining portion in which theconductivity-type-defining dopant is at a largely uniform firstconcentration and (b) a directly underlying drain-remote portion inwhich the conductivity-type-defining dopant is at a largely uniformsecond concentration significantly greater than, normally at least 10times greater than, the concentration of the conductivity-type-definingdopant in the drain-adjoining portion.

The concentration of the conductivity-type-defining dopant thenundergoes a step decrease, normally by at least a factor of 10, in goingfrom the drain-remote body-material portion up through thedrain-adjoining body-material portion to the drain. An n-channel IGFETprovided with this second type of drain-underlying hypoabrupt dopantprofile, along with the asymmetric channel-zone doping characteristicsof inventive structure A or A′, is generally referred to here asinventive structure E.

FIG. 67 generally illustrates the vertical dopant profiles through thedrains and into the underlying body regions for n-channel IGFETsconfigured as structures A/A′, B, and E. More particularly, thevariation of absolute dopant concentration N_(T) as a function of depthy along a vertical line through the drain is shown in FIG. 67 for ann-channel IGFET of each of structures A/A′, B, and E. Similar to what isshown in FIG. 56, concentration N_(T) of the total p-type dopant alongthe vertical line through the drain of the n-channel IGFET of structureE is at uniform concentration value N_(B0) from the upper semiconductorsurface to a depth y_(ST) equal to drain depth y_(D) plus distancey_(d0). Concentration N_(T) of the p-type dopant in the drain-adjoiningbody-material portion extending distance y_(d0) from depth y_(D) todepth y_(ST) is thus N_(B0). Distance y_(d0) is normally 0.05-1.0 μm,typically 0.1 μm.

At depth y_(ST), absolute dopant concentration N_(T) makes a step changefrom N_(B0) up to value N_(B1) greater than, normally at least a factorof 10 greater than, N_(B0). Concentration N_(T) of the p-type dopant inthe drain-remote body-material portion extending downward from depthy_(ST) is at value N_(B1) out to some depth beyond which theconcentration of the p-type dopant in the body material does not haveany significant effect on the characteristics, especially drain-to-bodycapacitance C_(DB), at the drain-body junction. Accordingly,concentration N_(T) of the p-type dopant in the body material makes astep decrease, normally by at least a factor of 10, in crossing from thedrain-remote body-material portion where concentration N_(T) of thep-type dopant equals N_(B1) up into the drain-adjoining body-materialportion where concentration N_(T) of the p-type dopant equals N_(B0) andthen remains at N_(B0) up to the drain-body junction.

FIG. 68 a illustrates an asymmetric long n-channel IGFET 500 configuredin accordance with the invention to implement structure E so as to beparticularly suitable for high-speed analog applications. IGFET 500 isarranged substantially the same as IGFET 170 of FIG. 18 a except thatbody material 108 consists of a heavily doped lower subsurface portion502 and an upper surface-adjoining portion 504 which extends to theupper semiconductor surface. P+ subsurface body-material portion 502underlies source 102, drain 104, and channel zone 106. The upperboundary (top) of subsurface body-material portion 502 is at depthy_(ST) below the upper semiconductor surface. Depth y_(ST) is normallyno more than 10 times, preferably no more than 5 times, drain depthy_(D). At its closest to source 102 and drain 104, subsurface portion502 is thus normally no more than 10 times deeper, preferably no morethan 5 times deeper, below the upper semiconductor surface than source102 and drain 104.

P-type surface-adjoining body-material portion 504 overlies and meets p+subsurface body-material portion 502. Channel zone 106 is part ofsurface-adjoining body-material portion 504. P+ pocket portion 120,which is shallower than source 102 here, is also part ofsurface-adjoining body-material portion 504. Item 124 in FIG. 68 a isnow the lightly doped material of surface-adjoining body-materialportion 504, i.e., the segment of portion 504 outside pocket portion120.

The p-type dopant in the segment of surface-adjoining body-materialportion 504 below drain 104 is present at a largely uniformconcentration equal to N_(B0). A typical value for concentration N_(B0)is 5×10¹⁵ atoms/cm³. The p-type dopant in the segment of subsurfacebody-material portion 502 below the preceding segment ofsurface-adjoining body-material portion 504 and thus below drain 104 ispresent there at a largely uniform higher concentration equal to N_(B1).Value N_(B1) is normally at least 10 times N_(B0), preferably at least20 times N_(B0), more preferably at least 40 times N_(B0), typically inthe vicinity of 100 times N_(B0).

FIG. 68 b illustrates another asymmetric long n-channel IGFET 501configured in accordance with the invention to implement structure E soas to be particularly suitable for high-speed analog applications. IGFET510 is arranged the same as IGFET 500 except that a subsurfaceelectrically insulating layer 512 typically consisting largely ofsilicon oxide contacts subsurface body-material portion 502 along itsbottom surface. In IGFET 510, the p-type dopant in the segment ofsubsurface body-material portion 502 underlying drain 104 from depthy_(ST) down to subsurface insulating layer 512 is largely uniformlydoped at concentration N_(B1).

An understanding of the hypoabrupt vertical dopant profile below drain104 in underlying body material 108 of IGFETs 500 and 510 is facilitatedwith the assistance of FIGS. 69 a-69 c (collectively “FIG. 69”), FIGS.70 a-70 c (collectively “FIG. 70”), and FIGS. 71 a-71 c (collectively“FIG. 70”). FIG. 69, which is generally analogous to FIG. 8, presentsexemplary dopant concentrations along vertical line 130 through source102 of IGFET 500 or 510. Exemplary dopant concentrations along verticallines 132 and 134 through channel zone 106 of IGFET 500 or 510 arepresented in FIG. 70 which are generally analogous to FIG. 9. FIG. 71,which is generally analogous to FIG. 10, present exemplary dopantconcentrations along vertical line 136 through drain 104 of IGFET 500 or510.

FIGS. 69 a, 70 a, and 71 a illustrate concentrations N_(I), alongvertical lines 130, 132, 134, and 136, of the individual semiconductordopants that form source 102, drain 104, subsurface body-materialportion 502, pocket portion 120 of surface-adjoining body-materialportion 504, and remainder 124 of portion 504. Concentrations N_(T) ofthe total p-type dopant and total n-type dopant in regions 102, 104,502, 120, and 124 along lines 130, 132, 134, and 136 are depicted inFIGS. 69 b, 70 b, and 71 b. FIGS. 69 c, 70 c, and 71 c depict net dopantconcentrations N_(N) along lines 130, 132, 134, and 136.

Curves/curve segments 102′, 102″, 102*, 104′, 104″, 104*, 120′, 120″,120*, 124′, 124″, and 124* in FIGS. 69-71 have the meanings presentedabove in connection with respectively analogous FIGS. 8-10. Curves 502′in FIGS. 69 a, 70 a, and 71 a indicate concentration N_(I), alongvertical lines 130, 132, 134, and 136, of the n-type dopant used to formsubsurface body-material portion 502. Curve segments 502″ in FIGS. 69 b,70 b, and 71 b represent concentration N_(T), along lines 130, 132, 134,and 136, of the total n-type dopant in subsurface portion 502. Curvesegments 502* in FIGS. 69 c, 70 c, and 71 c indicate concentrationsN_(N) along lines 130, 132, 134, and 136 of the net n-type dopant inportion 502.

Referring to FIG. 71 a, the p-type dopant in the portion of bodymaterial 108 below drain 104 of IGFET 500 or 510 has two primarycomponents referred to here as the “lower” p-type dopant and the “upper”p-type dopant. The lower p-type dopant is at high fixed concentrationN_(B1) in subsurface body-material portion 502 as indicated by curvesegment 502′. The upper p-type dopant is at low fixed concentrationN_(B0) in remainder 124 of surface-adjoining body-material portion 504as indicated by curve 124′. The upper p-type dopant is also present indrain 104 as indicated by the extension of curve 124′ into the areaencompassed by curve 104′.

The total p-type dopant in the portion of body material 108 below drain104 of IGFET 500 or 510 is indicated by the combination of curvesegments 502″ and 124″ in FIG. 71 b. As shown by the variation incombined curve 502″/124″, concentration N_(T) of the total p-type dopantin the portion of body material 108 below drain 104 substantiallyundergoes a step decrease in crossing from subsurface body-materialportion 502 at concentration N_(B1) into upper body-material remainder124 at concentration N_(B0) and then remains at concentration N_(B0) inmoving further upward to drain 104. Inasmuch as high concentrationN_(B1) is normally at least 10 times N_(B0), concentration N_(T) of thetotal p-type dopant in the portion of body material 108 below drain 104thus decreases hypoabruptly by at least a factor of 10 in moving upwardfrom subsurface body-material portion 502 through upper body-materialremainder 124 to drain 104.

High concentration value N_(B1) is, as mentioned above, preferably atleast 20 times N_(B0), more preferably at least 40 times N_(B0).Accordingly, the hypoabrupt decrease in concentration N_(T) of the totalp-type dopant in the portion of body material 108 below drain 104 ispreferably by at least a factor of 20, more preferably by at least afactor of 40.

FIG. 71 c shows that, as represented by the combination of curvesegments 502* and 124*, concentration N_(N) of the net p-type dopant inthe portion of body material 108 below drain 104 in IGFET 500 or 510,varies vertically in a similar manner to concentration N_(T) of thetotal p-type dopant in the portion of body material 108 below drain 104except that concentration N_(N) of the net p-type dopant in the portionof body material 108 below drain 104 drops to zero at drain depth y_(D),i.e., at drain-body junction 112. As in previously described IGFETs ofthe invention, the hypoabrupt dopant profile in the portion of bodymaterial 108 below drain 104 reduces parasitic capacitance alongdrain-body junction 112 of IGFET 500 or 510. Increased analog speed isthereby achieved for IGFETs 500 and 510.

Turning to the vertical dopant distribution below source 102 of IGFET500 or 510, curve segments 502′ and 124′ in FIG. 69 a have substantiallythe same shapes as in FIG. 71 a. Although curve 120′ appears in FIG. 69a, the total p-type dopant in the portion of body material 108 belowsource 102 consists of the lower and upper p-type dopants at respectiveconcentrations N_(B0) and N_(B1) because p-type pocket portion 120 isshallower than source 102 in the example of FIGS. 68 a and 68 b. Theportion of combined curve segments 502″ and 124″ at depth greater thansource depth y_(S) in FIG. 69 b is shaped substantially the same as theportion of combined curve segments 502″/124″ at depth greater than draindepth y_(D) in FIG. 71 b. Accordingly, concentration N_(T) of the totalp-type dopant in the portion of body material 108 below source 102changes hypoabruptly in largely the same way as concentration N_(T) ofthe total p-type dopant in the portion of body material 108 below drain104. Consequently, the parasitic capacitance along source-to-bodyjunction 110 is also reduced so as to further enhance the analogperformance of IGFET 500 or 510.

Similar to what occurs in IGFET 100 of FIG. 6, p-type pocket portion 120in IGFET 500 or 510 can be modified to extend deeper below the uppersemiconductor surface than source 102 and drain 104. In that case, thep-type pocket dopant in pocket portion 120 causes concentration N_(T) ofthe total p-type dopant in the portion of body material 108 below source102 to rise somewhat immediately below source-body junction 110 and thusbe somewhat greater than N_(B0) just below the bottom of source 102. Theparasitic capacitance along source-body junction 110 is higher than inthe example of FIGS. 68 a and 68 b but, with suitable choice for thedoping and depth of pocket 120, is still reduced. This again enhancesthe analog performance of IGFETs 500 and 510. Modifying pocket portion120 to extend deeper below the upper semiconductor surface than source102 and drain 104 does not have any significant effect on the draincharacteristics of IGFET 500 or 510 because substantially none of thep-type pocket dopant is then located in drain 104.

Channel zone 106 of IGFET 500 or 510 is asymmetrically longitudinallydoped in largely the same way as channel zone 106 of IGFET 170 in FIG.18 a. Since the dopant distributions of FIG. 7 and the associatedinformation presented above about FIG. 7 apply to IGFET 170, thisinformation generally applies to IGFETs 500 and 510. Punchthrough isthereby avoided in IGFETs 500 and 510. The channel length of IGFET 500or 510 can be reduced sufficiently to convert it into a short-channeldevice. In that case, the surface dopant distributions of FIG. 12 andthe associated information presented above about FIG. 12 generally applyto IGFETs 500 and 510.

Each S/D zone 102 or 104 of IGFET 500 or 510 can be modified to consistof main portion 102M or 104M and more lightly doped lateral extension102E or 104E. Alternatively or additionally, each S/D zone 102 or 104 ofIGFET 500 or 510 may include more lightly doped lower portion 102L or104L. In such cases, the dopant distributions presented in FIGS. 14, 16,and 17 and the associated information about those dopant distributionsgenerally apply to IGFETs 500 and 510 subject, in the case of FIGS. 16and 17, to replacing curves/curve segments 116′ and 114′, 116″ and 114″,and 116* and 114* respectively with curves/curve segments 502′, 502″,and 502*.

Further Complementary-IGFET Structures Suitable for Mixed-SignalApplications and Having Step Change in Body-Material DopantConcentration

FIG. 72 a illustrates another complementary-IGFET structure configuredaccording to the invention so as to be especially suitable formixed-signal applications. The complementary-IGFET structure of FIG. 72a is created from doped silicon material such as that of a bonded wafer.A patterned field region 520 of electrically insulating material extendsalong the upper surface of the silicon material to define a group oflaterally separated semiconductor islands, including islands 522 and524. Two asymmetric long-channel IGFETs 530 and 540 are formed along theupper semiconductor surface respectively at the locations of islands 522and 524.

IGFET 530 is an n-channel device which implements IGFET 510 of FIG. 68b. Source 102, drain 104, and channel zone 106 are situated in island522. Body material 108 of IGFET 530 consists of <100> p-typemonosilicon. A lower semiconductor layer 550 consisting of lightly doped<100> p-type monosilicon underlies and contacts insulating layer 512 sothat it is a subsurface layer. Field-insulating region 520, typically ofthe trench type, is vertically separated from subsurface insulatinglayer 512.

IGFET 540 is a p-channel device configured substantially the same asn-channel IGFET 500 of FIG. 68 a with the conductivity types reversed.IGFET 540 thus has a heavily doped p-type source 562 and a heavily dopedp-type drain 564 separated by a channel zone 566 of n-type body material568 consisting of a heavily doped lower subsurface portion 572 and anupper surface-adjoining portion 574 that extends to the uppersemiconductor surface. Source 562, drain 564, and channel zone 566 aresituated in island 524.

Body material 568 is formed with <100> n-type monosilicon. Subsurfaceportion 572 of n-type body material 568 extends over p− lowersemiconductor layer 550 and thereby forms a lateral pn junction 576 withsemiconductor layer 550. Subsurface body-material portion 572 also formsa vertical pn junction 578 with p+ subsurface body-material portion 502of IGFET 530. A reverse bias is applied across pn junction 578 toisolate IGFETs 530 and 540 from each other.

A heavily doped pocket portion 580 of n-type surface-adjoiningbody-material portion 574 extends along source 562 of IGFET 540. N+pocket portion 580 causes channel zone 566 to be asymmetricallylongitudinally dopant graded in a similar manner to the asymmetriclongitudinal dopant grading of channel zone 106 in IGFET 530. Item 584is the lightly doped n-type remainder of surface-adjoining body-materialportion 574. A gate dielectric layer 586, typically consisting primarilyof silicon oxide, overlies channel zone 566. A gate electrode 588 issituated on gate dielectric layer 586 above channel zone 566. Gateelectrode 588 extends partially over source 562 and drain 564. In theexample of FIG. 72 a, gate electrode 588 consists of very heavily dopedp-type polysilicon.

The n-type dopant in subsurface body-material portion 572 is present ata largely uniform concentration N_(B0)′. The n-type dopant in thesegment of surface-adjoining body-material portion 574 below drain 564is present in that segment at a largely uniform concentration N_(B1)′greater than N_(B0)′. Analogous to concentrations N_(B1) and N_(B0),concentration N_(B1)′ is normally at least 10 times N_(B0)′, preferablyat least 20 times N_(B0)′, more preferably at least 40 times N_(B0)′,typically in the vicinity of 100 times N_(B0)′. In the portion of bodymaterial 568 below drain 564, IGFET 540 thus has a hypoabrupt dopantprofile of generally the same nature that IGFET 530 has in the portionof body material 108 below drain 104. The vertical dopant profile in theportion of body material 568 below source 562 of IGFET 540 is likewiserespectively quite similar to the vertical dopant profile in the portionof body material 108 below source 102 of IGFET 530. Accordingly, IGFET540 has reduced parasitic capacitance along its drain-body andsource-body junctions.

FIG. 72 b illustrates a variation of the complementary-IGFET structureof FIG. 72 a. In the variation of FIG. 72 b, field-insulation region 520is provided with an electrically insulating extension 590, typically ofthe trench type, that reaches subsurface insulating layer 512. Thecombination of field-insulating region 520 and insulating extension 590laterally surrounds subsurface body-material portion 502 of IGFET 530.This dielectrically laterally isolates IGFETs 530 and 540 from eachother.

The conductivity types can be reversed in the complementary-IGFETstructures of FIGS. 72 a and 72 b. The resultant n-type body materialand n− lower semiconductor layer respectively corresponding to p-typebody material 108 and n− lower semiconductor layer 550 are then both<110> n-type monosilicon. The p-type body material corresponding ton-type body material 568 is <110> p-type monosilicon.

FIG. 72 c depicts a further variation of the complementary-IGFETstructure of FIG. 72 a. FIG. 72 d depicts a corresponding variation ofthe complementary-IGFET structure of FIG. 72 b. In the variations ofFIGS. 72 c and 72 d, a lower semiconductor layer 592 consisting oflightly doped <110> n-type monosilicon replaces p− lower semiconductorlayer 550. N-type body material 568 for p-channel IGFET 540 is formedwith <110> n-type monosilicon, rather than <100> n-type monosilicon, inthe complementary-IGFET structures of FIGS. 72 c and 72 d. P-type bodymaterial 108 for n-channel IGFET 530 continues to be <100> p-typemonosilicon in the complementary-IGFET structures of FIGS. 72 c and 72d.

The conductivity types can be reversed in the complementary-IGFETstructures of FIGS. 72 c and 72 d. In that case, the resultant p-typebody material and p− lower semiconductor layer respectivelycorresponding to n-type body material 568 and n− lower semiconductorlayer 592 are both <100> p-type monosilicon. The n-type body materialcorresponding to p-type body material 108 is <110> n-type monosilicon.

Manufacture of Further Complementary-IGFET Structures Having Step Changein Body-Material Dopant Concentration

The complementary-IGFET structure of FIG. 72 a is fabricated in thefollowing manner according to the invention. A structure is firstprovided in which (a) a subsurface semiconductor region consisting ofheavily doped <100> p-type monosilicon at high uniform concentrationN_(B1) adjoins a subsurface electrically insulating layer, (b) asurface-adjoining semiconductor region consisting of lightly doped <100>p-type monosilicon at low uniform concentration N_(B0) adjoins andoverlies the subsurface semiconductor region, and (c) a lowersemiconductor layer consisting of lightly doped <100> p-type monosiliconadjoins and underlies the subsurface insulating layer. The lightly dopedlower semiconductor layer constitutes p− lower semiconductor layer 550.

The initial structure can be created, for example, by bonding twosemiconductor wafers together through electrically insulating materialthat forms the subsurface insulating layer. One of the wafers has alightly doped <100> p-type monosilicon substrate that forms lowersemiconductor layer 550. The other wafer has a heavily doped <100>p-type monosilicon substrate and an overlying lightly doped <100> p-typemonosilicon epitaxial layer respectively doped substantially uniformlyat concentrations N_(B1) and N_(B0) to respectively form the subsurfacesemiconductor region and the surface-adjoining semiconductor region.

Field-insulating region 520 is formed along the outside (upper) surfaceof the p− surface-adjoining semiconductor region to define island 522for IGFET 530 and to define the location of island 524 for IGFET 540.Field insulation 520 may extend partially through the p−surface-adjoining semiconductor region so that field insulation 520extends deep into, but not fully through, p-type surface-adjoiningbody-material portion 504 in the completed complementary-IGFET structureas shown in FIG. 72 a. Alternatively, field insulation 520 may extendfully through the p-surface-adjoining semiconductor region and partiallyinto the underlying p+ subsurface semiconductor region. The portion ofthe p− surface-adjoining semiconductor region in island 522 constitutesa precursor to surface-adjoining body-material portion 504. Theunderlying portion of the p+ subsurface semiconductor regionsubstantially constitutes p+ subsurface body-material portion 502.

At the location for island 524, a cavity is formed through the p−surface-adjoining semiconductor region, through the underlying sectionof the p+ subsurface semiconductor region, and through the furtherunderlying section of the subsurface insulating layer down to p− lowersemiconductor layer 550. The remaining portion of the subsurfaceinsulating layer constitutes subsurface insulating layer 512. Heavilydoped <100> n-type monosilicon is epitaxially grown at uniformconcentration N_(B1)′ on the so-exposed section of lower semiconductorlayer 550 to substantially form n+ subsurface body-material portion 572.Lightly doped <100> n-type monosilicon is epitaxially grown at uniformconcentration N_(B0)′ in the cavity on subsurface portion 572 to form aprecursor to n-type surface-adjoining body-material portion 574.Body-material portion 572 and the precursor to body-material portion 574form island 524.

Gate dielectric layers 126 and 586 are respectively formed along theexposed (upper) surfaces of the precursors to p-type surface-adjoiningbody-material portion 504 for IGFET 530 and n-type surface-adjoiningbody-material portion 574 for IGFET 540. Gate electrodes 128 and 588 arerespectively formed on gate dielectric layers 126 and 586. N++ source102, n++ drain 104, and p+ pocket portion 120 are formed in theprecursor to surface-adjoining body-material portion 504. The remainingpart of the precursor to body-material portion 504 then substantiallyconstitutes portion 504 for IGFET 530. P++ source 562, p++ drain 564,and n+ pocket portion 580 are similarly formed in the precursor ton-type surface-adjoining body-material portion 574. The remaining n-typeportion of the precursor to surface-adjoining body-material portion 574then similarly substantially constitutes portion 574 for IGFET 540. Theoperations involved in forming gate electrodes 128 and 588, n++ source102, n++ drain 104, p+ pocket portion 120, p++ S/D zones 562 and 564,and n+ pocket portion 580 can be performed in various orders.

The complementary-IGFET structure of FIG. 72 b is fabricated accordingto the invention in the same way as the complementary-IGFET structure ofFIG. 72 a except that insulating extension 590 to field-insulatingregion 520 is formed in the p+ subsurface semiconductor region in thecourse of forming field insulation 520.

The complementary-IGFET structures of FIGS. 72 c and 72 d are fabricatedaccording to the invention in the same respective ways as thecomplementary-IGFET structures of FIGS. 72 a and 72 b except that alower semiconductor layer consisting of lightly doped <110> n-typemonosilicon replaces p− lower semiconductor layer 550. The lightly dopedlower semiconductor layer constitutes n− lower semiconductor layer 592.The initial structure used for creating the complementary-IGFETstructure of FIG. 72 c or 72 d can be created in the same way as theinitial structure used to create the complementary-IGFET structure ofFIG. 72 a or 72 b except that the first-mentioned wafer has a lightlydoped <110> n-type monosilicon substrate rather than a lightly doped<100> p-type monosilicon substrate.

Also, after forming the cavity through the p− surface-adjoiningsemiconductor region, through the underlying section of the p+subsurface semiconductor region, and through the further underlyingsection of the subsurface insulating layer down to n− lowersemiconductor layer 592, n+ subsurface body-material portion 572 isepitaxially grown on lower semiconductor layer 592 in the cavity asheavily doped <110> n-type monosilicon at concentration N_(B1)′. Theprecursor to re-surface-adjoining body-material portion 574 is thenepitaxially grown on n+ subsurface portion 572 in the cavity as lightlydoped <110> n-type monosilicon at concentration N_(B0)′.

Variations

While the invention has been described with reference to particularembodiments, this description is solely for the purpose of illustrationand is not to be construed as limiting the scope of the inventionclaimed below. For instance, silicon in the semiconductor body or/and ingate electrodes 128, 288, 328, 368, and 588 can be replaced with othersemiconductor materials. Replacement candidates include germanium, asilicon-germanium alloy, and Group 3a-Group 5a alloys such as germaniumarsenide.

Metal silicide layers can be provided along the upper surfaces ofsources 102 and 562, drains 104 and 564, and gate electrodes 128 and 588in IGFETs 530 and 540 of the complementary-IGFET structures of FIGS. 72a-72 d. Composite gate electrodes 128/258, 288/298, 328/338, and 368/378in IGFETs 210, 220, 230, 240, 380, and 390 of the complementary-IGFETstructures of FIGS. 29 and 30 or/and gate electrodes 128 and 588 inIGFETs 530 and 540 of the complementary-IGFET structures of FIGS. 72a-72 d can be replaced with gate electrodes consisting substantiallyfully of metal or substantially fully of metal silicide, e.g., cobaltsilicide or nickel silicide with dopant provided in the silicide gateelectrodes to control their work functions. Various modifications maythus be made by those skilled in the art without departing from the truescope of the invention as defined in the appended claims.

1. A structure comprising a field-effect transistor provided along anupper surface of a semiconductor body having body material of a firstconductivity type, the transistor comprising: a channel zone of the bodymaterial; first and second source/drain (“S/D”) zones situated in thesemiconductor body along its upper surface, laterally separated by thechannel zone, and being of a second conductivity type opposite to thefirst conductivity type so as to form respective pn junctions with thebody material, the first S/D zone comprising a main portion and a morelightly doped lateral extension laterally continuous with the mainportion and extending laterally under the gate electrode such that thechannel zone is terminated by the lateral extension and the second S/Dzone along the body's upper surface, the second S/D zone beingmaterially more lightly doped than the main portion of the first S/Dzone; a gate dielectric layer overlying the channel zone; and a gateelectrode overlying the gate dielectric layer above the channel zone;wherein: the body material is doped with semiconductor dopant of thefirst conductivity type; each pn junction reaches a maximum depth belowthe body's upper surface; the body material extends laterally under bothS/D zones; the dopant of the first conductivity type is present in bothS/D zones and has a concentration which locally reaches a mainsubsurface maximum concentration at a main subsurface body-materiallocation extending laterally below largely all of each of the channeland S/D zones; the main subsurface body-material location occurs no morethan 10 times deeper below the body's upper surface than the maximumdepth of the pn junction for each S/D zone; and the concentration of thedopant of the first conductivity type decreases by at least a factor of10 in moving upward from the main subsurface body-material locationalong a selected vertical line through a specified one of the S/D zonesto the body's upper surface.
 2. A structure as in claim 1 wherein theconcentration of the dopant of the first conductivity type decreases byat least a factor of 10 in moving from the main subsurface body-materiallocation along the vertical line to the pn junction for the specifiedS/D zone.
 3. A structure as in claim 2 wherein the concentration of thedopant of the first conductivity type decreases by at least a factor of20 in moving from the main subsurface body-material location along thevertical line to the pn junction for the specified S/D zone.
 4. Astructure as in claim 1 wherein the concentration of the dopant of thefirst conductivity type decreases substantially monotonically in movingfrom the main subsurface body-material location along the vertical lineto the pn junction for the specified S/D zone.
 5. A structure as inclaim 4 wherein the concentration of the dopant of the firstconductivity type decreases by at least a factor of 20 in moving fromthe main subsurface body-material location along the vertical linethrough the specified S/D zone to the body's upper surface.
 6. Astructure as in claim 1 wherein the specified S/D zone is the second S/Dzone.
 7. A structure comprising a field-effect transistor provided alongan upper surface of a semiconductor body having body material of a firstconductivity type, the transistor comprising: a channel zone of the bodymaterial; a source and a drain situated in the semiconductor body alongits upper surface, laterally separated by the channel zone, and being ofa second conductivity type opposite to the first conductivity type so asto form respective pn junctions with the body material, the sourcecomprising a main source portion and a more lightly doped lateral sourceextension laterally continuous with the main source portion andextending laterally under the gate electrode such that the channel zoneis terminated by the lateral source extension and the drain along thebody's upper surface, the drain being materially more lightly doped thanthe main source portion; a gate dielectric layer overlying the channelzone; and a gate electrode overlying the gate dielectric layer above thechannel zone; wherein: the body material is doped with semiconductordopant of the first conductivity type; each pn junction reaches amaximum depth below the body's upper surface; the body material extendslaterally under both the source and the drain; the dopant of the firstconductivity type is present in both the source and the drain and has aconcentration which locally reaches a main subsurface maximumconcentration at a main subsurface body-material location extendinglaterally below largely all of each of the channel zone, the source, andthe drain; the main subsurface body-material location occurs no morethan 10 times deeper below the body's upper surface than the maximumdepth of the pn junction for the drain; and the concentration of thedopant of the first conductivity type decreases by at least a factor of10 in moving upward from the main subsurface body-material locationalong a selected vertical line through the drain to the body's uppersurface.
 8. A structure as in claim 7 wherein the concentration of thedopant of the first conductivity type decreases by at least a factor of10 in moving from the main subsurface body-material location along thevertical line to the pn junction for the drain.
 9. A structure as inclaim 8 wherein the concentration of the dopant of the firstconductivity type decreases by at least a factor of 20 in moving fromthe main subsurface body-material location along the vertical line tothe pn junction for the drain.
 10. A structure as in claim 7 wherein theconcentration of the dopant of the first conductivity type decreasessubstantially monotonically in moving from the main subsurfacebody-material location along the vertical line to the pn junction forthe drain.
 11. A structure as in claim 10 wherein the concentration ofthe dopant of the first conductivity type decreases by at least a factorof 20 in moving from the main subsurface body-material location alongthe vertical line through the drain to the body's upper surface.